How can I add or change the menu?

Many email clients including Thunderbird offers the feature to add/remove/reorder the items in the toolbar/menu. How can this be achived in eM Client? E.g. I would like to have the language of the spellchecker as dropdown in the toolbar, because I have to switch the language very often.

This is not possible in eM Client.

That’s really sad. One of the big advantages of Thunderbird…