Should I apply with a non-subject LoR to a university?
Your recommenders should be able to vouch for your previous performance in undergrad and explain why you would be successful in a graduate program. A foreign language instructor might be able to provide the first part, but they might have more trouble with the second.
If you believe that your foreign language instructor could write you a strong letter highlighting characteristics that would make you successful in your program, then they might be a good person to ask. It will not necessarily harm your application to have a letter from such a person, but a letter from someone in your area might be stronger.