The correct terminology for such accents is Diacritics. After Googling this term, I found this function which is part of backbone.paginator. It has a very complete collection of Diacritics and replaces them with their most intuitive ascii character. I found this to be the most complete Javascript solution available today.

The full function for future reference:

function removeDiacritics (str) {

  var defaultDiacriticsRemovalMap = [
    {'base':'A', 'letters':/[\u0041\u24B6\uFF21\u00C0\u00C1\u00C2\u1EA6\u1EA4\u1EAA\u1EA8\u00C3\u0100\u0102\u1EB0\u1EAE\u1EB4\u1EB2\u0226\u01E0\u00C4\u01DE\u1EA2\u00C5\u01FA\u01CD\u0200\u0202\u1EA0\u1EAC\u1EB6\u1E00\u0104\u023A\u2C6F]/g},
    {'base':'AA','letters':/[\uA732]/g},
    {'base':'AE','letters':/[\u00C6\u01FC\u01E2]/g},
    {'base':'AO','letters':/[\uA734]/g},
    {'base':'AU','letters':/[\uA736]/g},
    {'base':'AV','letters':/[\uA738\uA73A]/g},
    {'base':'AY','letters':/[\uA73C]/g},
    {'base':'B', 'letters':/[\u0042\u24B7\uFF22\u1E02\u1E04\u1E06\u0243\u0182\u0181]/g},
    {'base':'C', 'letters':/[\u0043\u24B8\uFF23\u0106\u0108\u010A\u010C\u00C7\u1E08\u0187\u023B\uA73E]/g},
    {'base':'D', 'letters':/[\u0044\u24B9\uFF24\u1E0A\u010E\u1E0C\u1E10\u1E12\u1E0E\u0110\u018B\u018A\u0189\uA779]/g},
    {'base':'DZ','letters':/[\u01F1\u01C4]/g},
    {'base':'Dz','letters':/[\u01F2\u01C5]/g},
    {'base':'E', 'letters':/[\u0045\u24BA\uFF25\u00C8\u00C9\u00CA\u1EC0\u1EBE\u1EC4\u1EC2\u1EBC\u0112\u1E14\u1E16\u0114\u0116\u00CB\u1EBA\u011A\u0204\u0206\u1EB8\u1EC6\u0228\u1E1C\u0118\u1E18\u1E1A\u0190\u018E]/g},
    {'base':'F', 'letters':/[\u0046\u24BB\uFF26\u1E1E\u0191\uA77B]/g},
    {'base':'G', 'letters':/[\u0047\u24BC\uFF27\u01F4\u011C\u1E20\u011E\u0120\u01E6\u0122\u01E4\u0193\uA7A0\uA77D\uA77E]/g},
    {'base':'H', 'letters':/[\u0048\u24BD\uFF28\u0124\u1E22\u1E26\u021E\u1E24\u1E28\u1E2A\u0126\u2C67\u2C75\uA78D]/g},
    {'base':'I', 'letters':/[\u0049\u24BE\uFF29\u00CC\u00CD\u00CE\u0128\u012A\u012C\u0130\u00CF\u1E2E\u1EC8\u01CF\u0208\u020A\u1ECA\u012E\u1E2C\u0197]/g},
    {'base':'J', 'letters':/[\u004A\u24BF\uFF2A\u0134\u0248]/g},
    {'base':'K', 'letters':/[\u004B\u24C0\uFF2B\u1E30\u01E8\u1E32\u0136\u1E34\u0198\u2C69\uA740\uA742\uA744\uA7A2]/g},
    {'base':'L', 'letters':/[\u004C\u24C1\uFF2C\u013F\u0139\u013D\u1E36\u1E38\u013B\u1E3C\u1E3A\u0141\u023D\u2C62\u2C60\uA748\uA746\uA780]/g},
    {'base':'LJ','letters':/[\u01C7]/g},
    {'base':'Lj','letters':/[\u01C8]/g},
    {'base':'M', 'letters':/[\u004D\u24C2\uFF2D\u1E3E\u1E40\u1E42\u2C6E\u019C]/g},
    {'base':'N', 'letters':/[\u004E\u24C3\uFF2E\u01F8\u0143\u00D1\u1E44\u0147\u1E46\u0145\u1E4A\u1E48\u0220\u019D\uA790\uA7A4]/g},
    {'base':'NJ','letters':/[\u01CA]/g},
    {'base':'Nj','letters':/[\u01CB]/g},
    {'base':'O', 'letters':/[\u004F\u24C4\uFF2F\u00D2\u00D3\u00D4\u1ED2\u1ED0\u1ED6\u1ED4\u00D5\u1E4C\u022C\u1E4E\u014C\u1E50\u1E52\u014E\u022E\u0230\u00D6\u022A\u1ECE\u0150\u01D1\u020C\u020E\u01A0\u1EDC\u1EDA\u1EE0\u1EDE\u1EE2\u1ECC\u1ED8\u01EA\u01EC\u00D8\u01FE\u0186\u019F\uA74A\uA74C]/g},
    {'base':'OI','letters':/[\u01A2]/g},
    {'base':'OO','letters':/[\uA74E]/g},
    {'base':'OU','letters':/[\u0222]/g},
    {'base':'P', 'letters':/[\u0050\u24C5\uFF30\u1E54\u1E56\u01A4\u2C63\uA750\uA752\uA754]/g},
    {'base':'Q', 'letters':/[\u0051\u24C6\uFF31\uA756\uA758\u024A]/g},
    {'base':'R', 'letters':/[\u0052\u24C7\uFF32\u0154\u1E58\u0158\u0210\u0212\u1E5A\u1E5C\u0156\u1E5E\u024C\u2C64\uA75A\uA7A6\uA782]/g},
    {'base':'S', 'letters':/[\u0053\u24C8\uFF33\u1E9E\u015A\u1E64\u015C\u1E60\u0160\u1E66\u1E62\u1E68\u0218\u015E\u2C7E\uA7A8\uA784]/g},
    {'base':'T', 'letters':/[\u0054\u24C9\uFF34\u1E6A\u0164\u1E6C\u021A\u0162\u1E70\u1E6E\u0166\u01AC\u01AE\u023E\uA786]/g},
    {'base':'TZ','letters':/[\uA728]/g},
    {'base':'U', 'letters':/[\u0055\u24CA\uFF35\u00D9\u00DA\u00DB\u0168\u1E78\u016A\u1E7A\u016C\u00DC\u01DB\u01D7\u01D5\u01D9\u1EE6\u016E\u0170\u01D3\u0214\u0216\u01AF\u1EEA\u1EE8\u1EEE\u1EEC\u1EF0\u1EE4\u1E72\u0172\u1E76\u1E74\u0244]/g},
    {'base':'V', 'letters':/[\u0056\u24CB\uFF36\u1E7C\u1E7E\u01B2\uA75E\u0245]/g},
    {'base':'VY','letters':/[\uA760]/g},
    {'base':'W', 'letters':/[\u0057\u24CC\uFF37\u1E80\u1E82\u0174\u1E86\u1E84\u1E88\u2C72]/g},
    {'base':'X', 'letters':/[\u0058\u24CD\uFF38\u1E8A\u1E8C]/g},
    {'base':'Y', 'letters':/[\u0059\u24CE\uFF39\u1EF2\u00DD\u0176\u1EF8\u0232\u1E8E\u0178\u1EF6\u1EF4\u01B3\u024E\u1EFE]/g},
    {'base':'Z', 'letters':/[\u005A\u24CF\uFF3A\u0179\u1E90\u017B\u017D\u1E92\u1E94\u01B5\u0224\u2C7F\u2C6B\uA762]/g},
    {'base':'a', 'letters':/[\u0061\u24D0\uFF41\u1E9A\u00E0\u00E1\u00E2\u1EA7\u1EA5\u1EAB\u1EA9\u00E3\u0101\u0103\u1EB1\u1EAF\u1EB5\u1EB3\u0227\u01E1\u00E4\u01DF\u1EA3\u00E5\u01FB\u01CE\u0201\u0203\u1EA1\u1EAD\u1EB7\u1E01\u0105\u2C65\u0250]/g},
    {'base':'aa','letters':/[\uA733]/g},
    {'base':'ae','letters':/[\u00E6\u01FD\u01E3]/g},
    {'base':'ao','letters':/[\uA735]/g},
    {'base':'au','letters':/[\uA737]/g},
    {'base':'av','letters':/[\uA739\uA73B]/g},
    {'base':'ay','letters':/[\uA73D]/g},
    {'base':'b', 'letters':/[\u0062\u24D1\uFF42\u1E03\u1E05\u1E07\u0180\u0183\u0253]/g},
    {'base':'c', 'letters':/[\u0063\u24D2\uFF43\u0107\u0109\u010B\u010D\u00E7\u1E09\u0188\u023C\uA73F\u2184]/g},
    {'base':'d', 'letters':/[\u0064\u24D3\uFF44\u1E0B\u010F\u1E0D\u1E11\u1E13\u1E0F\u0111\u018C\u0256\u0257\uA77A]/g},
    {'base':'dz','letters':/[\u01F3\u01C6]/g},
    {'base':'e', 'letters':/[\u0065\u24D4\uFF45\u00E8\u00E9\u00EA\u1EC1\u1EBF\u1EC5\u1EC3\u1EBD\u0113\u1E15\u1E17\u0115\u0117\u00EB\u1EBB\u011B\u0205\u0207\u1EB9\u1EC7\u0229\u1E1D\u0119\u1E19\u1E1B\u0247\u025B\u01DD]/g},
    {'base':'f', 'letters':/[\u0066\u24D5\uFF46\u1E1F\u0192\uA77C]/g},
    {'base':'g', 'letters':/[\u0067\u24D6\uFF47\u01F5\u011D\u1E21\u011F\u0121\u01E7\u0123\u01E5\u0260\uA7A1\u1D79\uA77F]/g},
    {'base':'h', 'letters':/[\u0068\u24D7\uFF48\u0125\u1E23\u1E27\u021F\u1E25\u1E29\u1E2B\u1E96\u0127\u2C68\u2C76\u0265]/g},
    {'base':'hv','letters':/[\u0195]/g},
    {'base':'i', 'letters':/[\u0069\u24D8\uFF49\u00EC\u00ED\u00EE\u0129\u012B\u012D\u00EF\u1E2F\u1EC9\u01D0\u0209\u020B\u1ECB\u012F\u1E2D\u0268\u0131]/g},
    {'base':'j', 'letters':/[\u006A\u24D9\uFF4A\u0135\u01F0\u0249]/g},
    {'base':'k', 'letters':/[\u006B\u24DA\uFF4B\u1E31\u01E9\u1E33\u0137\u1E35\u0199\u2C6A\uA741\uA743\uA745\uA7A3]/g},
    {'base':'l', 'letters':/[\u006C\u24DB\uFF4C\u0140\u013A\u013E\u1E37\u1E39\u013C\u1E3D\u1E3B\u017F\u0142\u019A\u026B\u2C61\uA749\uA781\uA747]/g},
    {'base':'lj','letters':/[\u01C9]/g},
    {'base':'m', 'letters':/[\u006D\u24DC\uFF4D\u1E3F\u1E41\u1E43\u0271\u026F]/g},
    {'base':'n', 'letters':/[\u006E\u24DD\uFF4E\u01F9\u0144\u00F1\u1E45\u0148\u1E47\u0146\u1E4B\u1E49\u019E\u0272\u0149\uA791\uA7A5]/g},
    {'base':'nj','letters':/[\u01CC]/g},
    {'base':'o', 'letters':/[\u006F\u24DE\uFF4F\u00F2\u00F3\u00F4\u1ED3\u1ED1\u1ED7\u1ED5\u00F5\u1E4D\u022D\u1E4F\u014D\u1E51\u1E53\u014F\u022F\u0231\u00F6\u022B\u1ECF\u0151\u01D2\u020D\u020F\u01A1\u1EDD\u1EDB\u1EE1\u1EDF\u1EE3\u1ECD\u1ED9\u01EB\u01ED\u00F8\u01FF\u0254\uA74B\uA74D\u0275]/g},
    {'base':'oi','letters':/[\u01A3]/g},
    {'base':'ou','letters':/[\u0223]/g},
    {'base':'oo','letters':/[\uA74F]/g},
    {'base':'p','letters':/[\u0070\u24DF\uFF50\u1E55\u1E57\u01A5\u1D7D\uA751\uA753\uA755]/g},
    {'base':'q','letters':/[\u0071\u24E0\uFF51\u024B\uA757\uA759]/g},
    {'base':'r','letters':/[\u0072\u24E1\uFF52\u0155\u1E59\u0159\u0211\u0213\u1E5B\u1E5D\u0157\u1E5F\u024D\u027D\uA75B\uA7A7\uA783]/g},
    {'base':'s','letters':/[\u0073\u24E2\uFF53\u00DF\u015B\u1E65\u015D\u1E61\u0161\u1E67\u1E63\u1E69\u0219\u015F\u023F\uA7A9\uA785\u1E9B]/g},
    {'base':'t','letters':/[\u0074\u24E3\uFF54\u1E6B\u1E97\u0165\u1E6D\u021B\u0163\u1E71\u1E6F\u0167\u01AD\u0288\u2C66\uA787]/g},
    {'base':'tz','letters':/[\uA729]/g},
    {'base':'u','letters':/[\u0075\u24E4\uFF55\u00F9\u00FA\u00FB\u0169\u1E79\u016B\u1E7B\u016D\u00FC\u01DC\u01D8\u01D6\u01DA\u1EE7\u016F\u0171\u01D4\u0215\u0217\u01B0\u1EEB\u1EE9\u1EEF\u1EED\u1EF1\u1EE5\u1E73\u0173\u1E77\u1E75\u0289]/g},
    {'base':'v','letters':/[\u0076\u24E5\uFF56\u1E7D\u1E7F\u028B\uA75F\u028C]/g},
    {'base':'vy','letters':/[\uA761]/g},
    {'base':'w','letters':/[\u0077\u24E6\uFF57\u1E81\u1E83\u0175\u1E87\u1E85\u1E98\u1E89\u2C73]/g},
    {'base':'x','letters':/[\u0078\u24E7\uFF58\u1E8B\u1E8D]/g},
    {'base':'y','letters':/[\u0079\u24E8\uFF59\u1EF3\u00FD\u0177\u1EF9\u0233\u1E8F\u00FF\u1EF7\u1E99\u1EF5\u01B4\u024F\u1EFF]/g},
    {'base':'z','letters':/[\u007A\u24E9\uFF5A\u017A\u1E91\u017C\u017E\u1E93\u1E95\u01B6\u0225\u0240\u2C6C\uA763]/g}
  ];

  for(var i=0; i<defaultDiacriticsRemovalMap.length; i++) {
    str = str.replace(defaultDiacriticsRemovalMap[i].letters, defaultDiacriticsRemovalMap[i].base);
  }

  return str;

}

On OP's system, "ç" is length 3, (e.g. '0xc3', '0xa7', '0x00') thus it is not encoded as a single character.

A common encoding is UTF8: U+00E7 ç c3 a7 LATIN SMALL LETTER C WITH CEDILLA

Instead look for the string "ç" inside text_string and substitute with the shorter string "c".

You can try iconv, with the //TRANSLIT (transliteration) option

Ex. given

$ cat file
ë
ê
Ý,text
Ò
É

then

$ iconv -t ASCII//TRANSLIT file
e
e
Y,text
O
E

The suggestion above is excellent, but in this very moment it would not work because of an issue between Notepad++ and "Notepad++ Python Script". Since some months Notepad++ plug in manager downloads an old Python Script version that won't work with the editor. To fix that:

1. Exit Notepad++
2. Download the compatible version from SourceForge.
3. Run the downloaded installer by double clicking it. On newer Windows it'll ask to switch to Administrator privileges.
4. Make sure to pick the correct install drive at the beginning of the install process. It won't detect the Notepad++ installation disk correctly. I had to reinstall it again because by default it installs on C:\ even if Notepad++ is on another disk.
5. Follow the wizard instructions.
6. Once finished with the install process, (re)start Notepad++. Now go and open the Plugin menu. You should see a new "Python Script" item inside it. If it appears then you have force-installed the correct version well. You may also double check by opening the Plug in manager, going to the "Installed" tab and looking for an entry showing version 1.0.8 (at this time) of the Python plug in being present.

7. You are almost done. Go to the Plugins => Python Script => Show console menu. A pane shall appear at the bottom of Notepad++. It MUST show a prompt like the following:

   Python 2.7.6-notepad++ r2 (default, Apr 21 2014, 19:26:54) [MSC v.1600 32 bit (Intel)]
   Initialisation took 156ms
   Ready.
   

The various version numbers are current as of today, of course they shall change as time goes on. If the bottom pane shows an exception stating an exception occurred or (and) it stays blank, then you have installed a wrong Python plug in version.

Now, let's apply the script in the correct way:

1. Open two new, blank tabs/files.
2. Paste your accented text in the first.
3. Right click the tab of the second and select the 'Move to Other View' menu. The Notepad++ windows will split.
4. Open the Python Script console as explained above (Plugins => Python Script => Show console menu).
5. Go to the console line at the bottom of the Python pane, it has a ">>>" marking at its beginning.
6. Type: from Npp import * and then press Enter (from now assume you'll always press Enter at the end of the commands).
7. Enter: import unicodedata in the same input text.
8. Click (select) the tab containing the accented text (this is important!).

9. Enter the following commands, one line at a time, in the Python prompt and then press Enter after each line:

   eText = editor.getText()
   uText = unicode(eText, "UTF-8")
   nText = unicodedata.normalize( "NFKD", uText )
   

If you want to be sure Python "really got the text in": after you typed eText = editor.getText() (+ Enter key), enter: print eText + Enter. You should see your accented text dumped in the Python console output pane.

10. Click (select) the empty tab (this is important!).
11. Enter: editor.addText( nText.encode('ASCII', 'ignore') ) in the usual Python console command input text box.
12. The empty tab shall fill in with the converted, accent-less text. Make sure to follow this to-do list carefully because it's easy to miss a step (expecially clicking the tabs) and then you'll have to restart from scratch.

I don't know if it will manage all the non-ascii characters in your input file, but get-char-code-property is able to deal with all the cases you show. The doc string C-h v get-char-code-property says:

(get-char-code-property CHAR PROPNAME)

Return the value of CHAR’s PROPNAME property.

The property you want is decomposition which somehow figures out that accented characters are made up from a base character and an accent. The call (get-char-code-property c 'decomposition) then returns a list with one or two elements: the first is the base character and the second is the accent.

Here are some examples of calling it (note that ?n is the character n - or equivalently the integer 110, since Emacs represents characters by integers):

(get-char-code-property ?a 'decomposition) --> (97)
(get-char-code-property ?n 'decomposition) --> (110)
(get-char-code-property ?e 'decomposition) --> (101)

(get-char-code-property ?á 'decomposition) --> (97 769)
(get-char-code-property ?ñ 'decomposition) --> (110 771)
(get-char-code-property ?ê 'decomposition) --> (101 770)

As you can see the first element of the list is the unaccented character (or integer). If you are wondering what characters the integers 769, 770 or 771 represent, you can use the same function with the name property:

(get-char-code-property 770 'name) -->  "COMBINING CIRCUMFLEX ACCENT"

COMBINING characters are combined with the previous character to produce the accented (or otherwise decorated) compound character.

So all you have to do is to loop over all characters of your string, run them through get-char-code-property with the decomposition property and throw away everything but the first character which is the base character. Here's e.g. a simple function that takes a string and translates it:

(defun xlate-unaccented (s)
  (mapconcat
   (lambda (c)
     (char-to-string
      (car (get-char-code-property c 'decomposition))))
   s ""))

The last argument to mapconcat is the separator argument (here an empty string). That became optional at some point after 28.1 but, as the OP points out in a comment, it is necessary in 28.1 or earlier (and maybe some later version too): you can also specify nil instead of the explicit "", but the argument has to be present.

Here are some tests:

(xlate-unaccented "El Niño") --> "El Nino"
(xlate-unaccented "René")  --> "Rene"
(xlate-unaccented "tåg") --> "tag"

Note that this is going to fail for more complicated characters, e.g. characters with multiple accents. For example, consider the character ậ whose name is LATIN SMALL LETTER A WITH CIRCUMFLEX AND DOT BELOW. If you evaluate (get-char-code-property ?ậ 'decomposition), you will get (7841 770) whose base character is NOT unaccented: instead it's the character with name "LATIN SMALL LETTER A WITH DOT BELOW". You need to apply the decomposition again: (get-char-code-property 7841 'decomposition) --> (97 803) to come up with the unaccented 97 (aka a):

The xlate-unaccented function strips one accent:

(xlate-unaccented "La Niñậ") --> "La Ninạ"

Running it through again (and again) will get rid of the remaining accent(s). You can also loop inside the lambda as @gigiair points out in a comment:

(defun xlate-unaccented (s)
  (mapconcat
   (lambda (c)
     (char-to-string
      (let ((dec (get-char-code-property c 'decomposition)))
        (while (cdr dec)
          (setq dec (get-char-code-property (car dec) 'decomposition)))
        (car dec))))
   s ""))

BTW, in case you are wondering: I didn't know about this function at all. I found it by noticing the decomposition field in the output of C-u C-x = (which ends up calling describe-char). So I invoked C-h f describe-char and clicked on the source link; scanning through the code of the function, I found get-char-code-property called near the bottom of it and did C-h f get-char-code-property, but I also had to look at C-h v describe-char-unidata-list: describe-char calls get-char-code-property on each element of that list; its default value is (name old-name general-category decomposition), but you can customize it to add more properties for describe-char to display. The customization buffer for it provides a convenient list of all the properties.

I didn't dig into it enough to see why the . was being replaced with A, but this version doesn't have that behaviour. It doesn't work exactly the same and it operates on the whole string, but that likely isn't a problem unless it's used in a large textfield, in which case it could be optimised other ways.

const com_acento = 'áàãâäéèêëíìîïóòõôöúùûüçÁÀÃÂÄÉÈÊËÍÌÎÏÓÒÕÖÔÚÙÛÜÇ<,>´`-,~';
const sem_acento = 'aaaaaeeeeiiiiooooouuuucAAAAAEEEEIIIIOOOOOUUUUC          ';

function retiraAcento(string) {
    return string
    .split('')
    .map(char => {
        const charIdx = com_acento.indexOf(char)
        if (charIdx !== -1) {
        return sem_acento[charIdx]
      }
      return char
    })
    .join('')
}

function replaceCharOnChange(evt) {
  event.target.value = retiraAcento(evt.target.value)
}

document.querySelector('input').addEventListener('keyup', replaceCharOnChange)

Apologies it's half rewritten in english! I'd also look into using another data structure than the two same-length strings. The simplest would be an object lookup table eg:

{á: 'a', ç: 'c', ...}

but there's other ways you could go about it too

I've not used this method, but Michael Kaplan describes a method for doing so in his blog post (with a confusing title) that talks about stripping diacritics: Stripping is an interesting job (aka On the meaning of meaningless, aka All Mn characters are non-spacing, but some are more non-spacing than others)

static string RemoveDiacritics(string text) 
{
    var normalizedString = text.Normalize(NormalizationForm.FormD);
    var stringBuilder = new StringBuilder(capacity: normalizedString.Length);

    for (int i = 0; i < normalizedString.Length; i++)
    {
        char c = normalizedString[i];
        var unicodeCategory = CharUnicodeInfo.GetUnicodeCategory(c);
        if (unicodeCategory != UnicodeCategory.NonSpacingMark)
        {
            stringBuilder.Append(c);
        }
    }

    return stringBuilder
        .ToString()
        .Normalize(NormalizationForm.FormC);
}

Note that this is a followup to his earlier post: Stripping diacritics....

The approach uses String.Normalize to split the input string into constituent glyphs (basically separating the "base" characters from the diacritics) and then scans the result and retains only the base characters. It's just a little complicated, but really you're looking at a complicated problem.

Of course, if you're limiting yourself to French, you could probably get away with the simple table-based approach in How to remove accents and tilde in a C++ std::string, as recommended by @David Dibben.