Misha - You _can_ have multiple languages in one course (and the user can choose between them using the Language Selector plugin), but only in standalone Framework projects it seems.
Trying to re-import a course with multiple language content to the Authoring Tool seems to discard all but the first language (in alphabetical order?), ignoring even the
_defaultLanguage set in the course.json configuration. If there's a way to get the Authoring Tool to keep the extra language data on import that'd make it a lot easier to maintain an "authoritative" version of a multi-language course, so updates can start in one place (even if content updates have to round-trip through an export-translate-reimport workflow)