Hello Rob,
there are no plans to make the Menu bar customizable.
Technically ther eis only 2 clicks, as you need to click Menu, then point to Tools>Spellcheck and then finally click your chosen language.
If you switch spell-check languages often, you might consider changing your spell-checker depending on your keyboard settings, perhaps.
In main window’s Menu>Tools>Settings>General>Spell-checker you can set the Mail Spell-Checker to the Keyboard language option and when you switch Keyboard language the Spell checker will change automatically with the langauge change shortcut.
Just download the needed Keyboard language settings available in Windows, if you don’t have it already.
Okay, technically speaking you are correct. Conclusion that more actions and clicking is to be done remains. I think this is not an issue you will encounter as you do all your correspondence in English but when you do 50% English and 50% another language, switching language frequently is annoying.
I appreciate you propose an alternative solution. However, changing language will mixed up the special characters like: @#$$%;:[]{} etc.
In the event you had no plans in this direction, please see this as request to have the menu bar back or at least leave this choice with the user.