I've just made a test on a Facebook test page, with 1 test user liking that page. I set the language of this User 1 to French.
Then I posted 6-7 time with various combinations of multilingual posts. Here are my findings:
First posts translated in both English and French showed well on the wall of User 1 only in French, no English post was posted. One later post however showed in English instead of French on the wall of User 1.
Thinking that posting one by one in each language restricting the languages to one per post either English or French would be a solution, well it isn't. The untranslated posts in English show on the wall of User 1.
Is there any solution for this, or can anybody else confirm my findings. I consider these Facebook bugs and unfortunately the multilingual posts on Facebook are pretty buggy.