Students can often take sign language instead of an oral language to fulfill foreign language requirements. If your high school does not offer sign language, you can typically take it at a local junior college and transfer the credits. Just be sure to speak with your academic counselor to make sure specific credits will be accepted.
My daughter is 15 and is in ninth grade at an online public school using college prep curriculum. She is dyslexic and has A.D.H.D. Considering her struggles with language, I think studying a foreign language would not benefit her. What have other parents of dyslexic students done, or offered, instead?