Should I participate in language olympiads if I want to study chemistry at a university?

Just because you want to study chemistry in a university doesn't mean that you can't pursue other interests. You should participate in language Olympiads if it's your passion. However, if it's just a suggestion from someone for building a resume and you don't like it, you'd rather opt out.

In the US system you typically apply for a university, and not a major. As a general rule, you cannot really "choose" to study chemistry in your application; you can only indicate your interest to pursue chemistry as a major later on. Seeing as this is the case, if you genuinely enjoy linguistics, I say go for it. You can choose to not include the linguistics olympiads later on in your application if you think that they don't add anything to your portfolio, but I think they'll make you a more interesting applicant.