Changing language of the user interface requires a trip to Admin > Personal > Settings > Language
Have a pulldown language menu somewhere convenient
Multilingual teams can work together better. (I don't know how many user teams are multilingual...)
For exactly this use case (as I'm switching between Languages often for screenshots in explanations) I created this plugin a long time ago and am quite happy with it since (apart from the fact that the name is misspelled):
Of course, it is a bit more obvious than a dropdown as it adds dedicated buttons for every selected language to the main navigation bar.