I think the way the sentence is organized is making this confusing. I would say, "Speakers of which language find it easiest to learn French?"

Both sentences are incorrect considering the meaning you intended. The correct form would be "Speakers of which language find it easiest to learn French?" or "Native speakers of which language find it easiest to learn French?"

Hey Hernandez, The first sentence is correct. In the second you're not using the apostrophe in the correct way. There are a number of reasons to use an apostrophe in English. I've put a link below to help you understand it more. I hope this helps :) https://grammar.yourdictionary.com/punctuation/apostrophe-rules.html