There is just an English version, and you can change which spell correcting dictionaries it uses.

To change the dictionary being used, go into the options.

Thunderbird menu: Tools: Options: Composition section: Spelling tab (If you cannot see the Thunderbird menu, press the Alt key or F10 key on your keyboard to show the menu)

Click on "Download More Dictionaries" and download the "English (British)" dictionary.

There is actually a GB locale and a GB version. As such the version is fully localized, or it should be, with proper British English spellings

See https://www.mozilla.org/en-US/thunderbird/all/

I am not aware of a display in the user interface anywhere of the actual locale, but there is a hidden preference which you can find in the config editor called general.useragent.locale which is the locale Thunderbird uses.