What is the difference between similarity and similitude?

I must say : French language have many similarities with English or I must say : French language have many similitudes with English? Thank you for your answer!

--

Similitude is not a correct word, or at least one that I have never used or heard. Also, you don't need to use the word language as it is already understood that this is what you are referring to. (If you mean people, you would say 'French people...' or 'The French...')

So, the best ways to say it are:

French has many similarities with English.

French is (quite/very) similar to English - though this is a bit stronger in meaning than the sentence above.