The person who submits the first post in a thread selects the category and forum where the thread will be created. Each forum is associated with exactly 1 language. So if the user chooses an en-us forum, he or she should post the question
in English. If they choose a Turkish forum, the post should be in Turkish. And so forth...
There is no language detection logic in the Forums "Ask a Question" page - we rely on the user to choose the target forum, and that determines the target language.