It can be hard to squeeze in extra courses at BS. Arts will depend on how the program is structured; for instance, at Mercersburg, students can’t participate in the major extracurricular music groups unless they are also enrolled in the corresponding music classes.
On the original topic: Mercersburg has an excellent German teacher and a longstanding exchange relationship with a German gymnasium. Extraordinary French teacher, too, btw, can’t comment on others.
College may allow for more language exploration. My daughter is in a program with a ton of restrictive requirements, but because of her interest, she has continued French (currently studying in Paris), has started learning Spanish (and is continuing her Spanish study this semester - in French), and has picked up bits of other languages using Duolingo and her friends. She goes to a school where she is allowed to take too many classes lol. However, my son goes to a college that pretty strictly limits students to four classes per semester, and he has only been able to take one German course and probably won’t take any more.