Render a menu block in current user's language
You don't need a custom module for this.

Drupal can have seperate language detection for interface (menu item/interface elements) and content.  There was a bug that was fixed recently so make sure you are on the latest 8.6.x version.

Assuming you have enabled all 4 core language modules and you have enabled at least 1 extra language than configure the language detection methods to accomplish what you want as I its outlined below.

Go to Configuration &gt; Regional and language &gt; Languages &gt; Detection and selection and 


enable Account administration pages. This will allow each user to choose their preferred interface language (there will be a new option when they edit their profile page)

enable content language detection. This will make available the language switcher (content)) block.
enable 1 detection method.  In my case I choose url (i.e. /en /fr) but you can choose any other or more methods if you like.


ΝΟΤΕ: The detection methods take precedence as they appear from top to bottom provided they are also enabled.



Go to Structure &gt; Block layout and enable language switcher (content) (not language switcher (interface text)

Now lets say a user has chosen English as their interface language they can press on the language switcher (content) and their interface (menu, admin menu etc) will always  stay English.