You did search the forums and docs for a possible answer before asking the question though eh?
https://docs.moodle.org/311/en/Language_FAQ#Warning_message:_Your_server_does_not_seem_to_fully_support_the_following_languages:
https://moodle.org/mod/forum/discuss.php?d=390750
https://tracker.moodle.org/plugins/servlet/mobile#issue/MDL-62728
https://docs.moodle.org/311/en/Language_FAQ#Warning_message:_Your_server_does_not_seem_to_fully_support_the_following_languages:
https://moodle.org/mod/forum/discuss.php?d=390750
https://tracker.moodle.org/plugins/servlet/mobile#issue/MDL-62728