I seem to recall hearing that French francophones say the French equivalent of "in three days" where anglophones say "in two days". (I don't speak French, and can't vouch for it.) That, coupled with a discussion in the Talmud (M'nachos 77:1), makes me wonder:
Is there a language of which the following is known? In such a language, wording equivalent to "to add a fifth" or "ten percent more" or the like means the fraction is computed as a fraction of the total (including the fraction). Thus, for example, "add a fifth to 100" would yield 125 and "ten percent more than ninety" would be 100.