Because of the long & complicated history of French in English, there are thousands of English words & phrases that are obviously of French origin but have dangerously different meaning. They are called faux amis (false friends), and whole dictionaries of them have been published. They sometimes trip up even people who are fluent in both languages when they are translating something and the tempting mistranslation is right in front of them.

demander, not demand but request
en effet, not in effect but indeed
librairie, not library but bookstore
morale (n.), not morale but moral (of a story)
moral (n.), not moral but morale (of troops etc.)
enfant, not infant but child

The divergences between British & American English can also sometimes cause embarrassment: Look up "table (a motion)", "knock up", "screwed" (slang), & "post" (verb).