One possibility is to avoid having dead keys and type accented letters with a Compose key instead. For example, type Compose " a to enter ä. You'll have to choose a keyboard layout option that includes a Compose key; a common choice is the key to the left of the right Ctrl key (which I think Windows calls Menu). The advantage of Compose is that you get to be able to type many fancy characters without changing your main layout. The disadvantage is that it takes three presses instead of two for an accented letter.

Alternatively, you can configure the effect of dead keys by creating a file called .XCompose (note CApitalization) in your home directory and listing the combinations you want. Something like:

include "/usr/share/X11/locale/en_US.UTF-8/Compose"

<dead_acute> <space> : "'" apostrophe
<dead_acute> <exclam> : "'!"
<dead_acute> <quotedbl> : "'\""
# etc...
<dead_acute> <a> : "á"
<dead_acute> <b> : "'b"
# etc...

The syntax is fairly simple: the sequence of keys to the left of the : is turned into the string between "" on the right of the :. The extra word on the right is the keysym (i.e., the name of the key) corresponding to this sequence of keys; it's not terribly important. Anything from a # to the end of the line is ignored.

Change the first line to point to the system file if your distribution puts it in a different place. You can look in this file for more syntax examples. <Multi_key> is another name for the Compose key.

On the left, the names between <angle brackets> are keysyms. You can find the list of keysyms in /usr/include/X11/keysymdef.h (this file is in a development package, e.g., x11proto-core-dev on Ubuntu).

Yes, that's a lot of typing, but you should be able to automate much of it by copy-pasting chunks of keysymdef.h and doing a few clever mass replacements. Something that will help is that you can reuse the hexadecimal code on the right to make the right-hand string thanks to the "\xdd" syntax: turn e.g.

#define XK_exclam                        0x0021  /* U+0021 EXCLAMATION MARK */

into

<dead_acute> <exclam> : "'\x21"

Finally, you can set up a keyboard layout with dead keys and a Compose key. Being German, you might set up only " as a dead key. If you're going that route, the simplest option is to use the GUI to set up a keyboard with no dead key and an .Xmodmap file for the dead key, containing something like

keysym apostrophe = apostrophe dead_quotedbl

Complementing Vítor Souza answer

Using an US international keyboard layout with OS Language in English and having a beautiful cedilla.

To have English Language with ç/Ç working beautifully ( 'c -> cedilla ) isn't necessary to change the compose files. What one have to do is to set the LC_CTYPE with a locale that contains this definition:

<dead_acute> <c>            : "\347"    ccedilla.

Like pt_BR.UTF-8 (Brazilian Portuguese / Português Brasileiro).

X keyboard map:

# setxkbmap -layout us -variant intl

/etc/locale.conf

# nano /etc/locale.conf
LANG=en_US.UTF-8
LANGUAGE="en_US"
LC_CTYPE=pt_BR.UTF-8

Update locale

# source /etc/profile

And for tty:

Set the right keymap, font and font map, eg.

# nano /etc/vconsole.conf
KEYMAP=us-acentos
FONT=ter-114n
FONT_MAP=8859-1

Here's how I do it on Debian (with X, xdm and fvwm, not Wayland):

May keyboard identifies as "pc104/us/altgr-int" under X.

I have an .Xmodmap file, which contents include

remove mod4 = Super_L Hyper_L
keysym Super_L = Mode_switch
keycode  34 = bracketleft braceleft adiaeresis Adiaeresis
keycode  35 = bracketright braceright udiaeresis Udiaeresis
keycode  51 = backslash bar odiaeresis Odiaeresis
keycode  21 = equal plus ssharp ssharp
keycode  47 = semicolon colon doublelowquotemark singlelowquotemark paragraph degree paragraph degree
keycode  48 = apostrophe quotedbl leftdoublequotemark leftsinglequotemark dead_acute dead_diaeresis dead_acute dead_diaeresis

This allows me to use the left "Windows" key as a mode switch to get äüöÄÜÖß on the places where those are normally on a German keyboard, and additionally „“‚‘ on the middle row.

No Alt-Gr needed (I always break my finger when I try to use that), and Shift works as expected for Umlauts.

The .Xmodmap is read by a command xmodmap $HOME/.Xmodmap in the ~/.xsession file that xdm executes on session login.

There are lots of variants possible; in particular Arch Linux and the display manager your friend uses (whatever he has installed, or whatever Arch uses by default) will likely require slightly different file names, or a different setup.

Use the English (US, Alternative international) layout.

Which distro are you using? At least on Debian-based distros, the keyboard is controlled by /etc/default/keyboard.

See the keyboard(5) man page for more information: Link