I see. Thanks Kaba. I just tried on my Moodle 3.5 test installation and I can't reproduce your problem. In my case, whenever I change the language, the Main menu items switch to use the expected variant instantly, without the need to re-login.
- What Moodle version are you using?
- Are you using the default Multilang filter or some contributed add-on?
- What languages do you have installed? What syntax do you use for those items?
- On page Site administration > Plugins > Caching > Configuration, what cache store do you have configures for the cache "List of available languages" ?