I don't know if that's what's bothering you, but for quatre-vingts, forget what the name means. Don't make 4 x 20 in your head, you'll mix everything up. Quatre-vingts is 80, that's all. Same for 70 and 90. For the rest... it just takes practice. Everybody struggles with numbers in foreign languages (I'm having a stroke each time I see something like neunhundertneunundneunzig) Edit : thank you for my first reddit award ! More on reddit.com

Yes, thousand separators go between every 3 numbers, just like in English. Both periods and spaces are used as thousand separators, it depends on the house style of any organisation, or personal habits. For example the French tax administration writes numbers as 12 539,26 € while the Belgian one uses 12.539,26 EUR. It's also common for numbers not to be separated in context where you rarely deal with numbers over 4 digits, and for the currency sign to serve as the decimal separator in shop price listings. For example, 1234€69 or 1234€69 for what would be written in English as €1,239.69. More on reddit.com

Québec counts the same as the French though. It’s supposedly an influence from Gaulish, a Celtic language once spoken in France before the arrival of the Romans introducing Latin. Celtic languages still count like that today. Sous l'influence des langues pré-indo-européennes, le français conserve des vestiges jusqu'à nos jours, dans les numéraux[4]. Au Moyen Âge, on trouve ainsi les formes vingt et dix (30), deux vingt (40), deux vingt et dix (50), trois vingt (60), etc.[5]. Une ancienne attestation, « VII vinz liverez et IIII », dans les Lois de Guillaume le Conquérant, au xie siècle, peut laisser penser à une origine normande, le système vicésimal se retrouvant aussi en danois. Les linguistes penchent cependant traditionnellement pour un héritage du gaulois, les langues celtiques modernes possédant ou ayant possédé une numération par vingt. Mais les dizaines supérieures à vingt sont en réalité inconnues en gaulois, à l'exception d'une forme, peut-être latinisée, pour « trente », or cette forme n'est pas vicésimale. L'hôpital des Quinze-Vingts, fondé à Paris en 1260, qui accueillait à l'origine trois cents aveugles, témoigne également de cet usage. Dès la fin du Moyen Âge, il est progressivement supplanté par le système décimal - trente, quarante, cinquante, soixante. Mais pour les trois dernières dizaines précédant cent, cette évolution ne s'est pas partout diffusée de la même façon dans la francophonie au cours de l'époque moderne. https://fr.m.wikipedia.org/wiki/Système_vicésimal French did partially switch to decimal except for 70, 80, and 90, which didn’t fully become decimal throughout the French-speaking world for some reason. Hence why the Belgians and Swiss are particular like that. Edit: also this http://monsu.desiderio.free.fr/curiosites/septante.html More on reddit.com

Languages have many different types of [numeral systems]( http://en.wikipedia.org/wiki/Numeral_(linguistics) - there are much "weirder" systems than French described in the article. English happens to have a fairly straightfoward one - a base-10 (decimal) system throughout. French looks funny to English speakers because it uses both a base-20 (vigesimal) system (for numbers 70-99) and a decimal system. This is likely from some sort of contact with Celtic . It is not cross-linguistically uncommon for parts of numeral systems to be borrowed with these kinds of results - Danish and Georgian, for example, have similar situations. More generally, natural languages just don't evolve to be logically optimal; I don't know why you have that expectation. Language has social as well as communicative functions, and language changes like borrowing sometimes look "pointless" or "complicated" on the surface, but simply have language-external motivations. English, for example, borrowed many words from French that simply replaced pre-existing Germanic forms. This was not for any good language-internal reason, and it certainly didn't make our language any simpler or more efficient, but happened purely for reasons of social prestige and politics. In the end, all languages are equally full of redundancies, exceptions, and quirks. The notion that French is more beautiful, pure, or pleasing than any other language has its roots in culture-specific language ideologies and not in any facts about the scientific realities of the language. More on reddit.com