From Joomla! Documentation

Some (but not all) multilingual sites experience a problem where all content not in the default language is not displayed and the language filter module does not seem to work.

An official fix is not yet announced. However there were two changes that may be causing this issue and users may want to try fixing these. Try each fix in order and test to see if it resolves the problem.

First, if you have contact data you must ensure that every contact has a language (if it says "Undefined" in the contact manager, that tells you no language is assigned) and every contact that has a linked user should either have a version set to language ALL or else have one version set to each content language used on your site. That is if you have 3 languages you must either have 3 separate contacts or you must have one set to all.

Second, open the language filter plugin, make sure the cookie lifetime is set to year and save.