Steps to reproduce the issue

A multilingual site. Create a menu item MENU_A in language LANG_A pointing to a category CAT, another menu item MENU_B in language LANG_B pointing to the same category CAT.
Create an article with language all inside CAT.

Go to frontend, select LANG_A in laguage switcher module, select the menu item MENU_A, you will see the article in LANG_A. Click on that article.
Now look at the language switcher module:

Actual result

the flag link for LANG_B points just to MENU_B since it asks for content association and since there is no explicit association of the article to LANG_B (getAssociations).
.

Expected result

The flag link in the language switcher for LANG_B should point to the current article with MENU_B.
Language switcher module and language filter plugin should treat an item (at least an articles) with language "all" as they have implicit associations to any language, since the version of an article in another language is the article itself.
This can be accomplished but checking article language in mod_languages and language filter plugin or modifying getAssociations() function to return association also for item that do not have one but are themself multilingual.

System information (as much as possible)

Joomla 3.8.13

Additional comments