User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

Set composition to be spell checked in en, de, es in the preferences.
Start a new composition, you get en, de, es.
Switch off German in the compose window.
Save the draft, close the compose window, check the draft and see:
Content-Language: en-GB, es-ES
Edit draft.

Actual results:

In the spelling button and the status bar, en and es are restored.
However, right click in the message body, Languages, de is also selected.
There a variations on this, see below.

Expected results:

The chosen languages should be restored everywhere consistently.

Variation for TB 102: Set settings to single language en, in new composition select de, es, deselect en. Save draft, close window, edit draft. In the body, the language shows as en, elsewhere as de, es.

xref bug 1678812. Not sure how useful it is to set Content-Language to more than one language only because those dictionaries were enabled.

Actually, nothing is restored, the menus look OK, but the language(s) they display aren't the one(s) which is uses for spell checking.
Hacky solution here:
https://github.com/Betterbird/thunderbird-patches/blob/main/102/bugs/1775376-fix-dict-edit-draft.patch

(In reply to Magnus Melin [:mkmelin] from comment #1)

xref bug 1678812. Not sure how useful it is to set Content-Language to more than one language only because those dictionaries were enabled.

Sadly Content-Language is our way to store active dictionaries. Sending that out in the final message is a separate debate.

Actually, nothing is restored, the menus look OK, but the language(s) they display aren't the one(s) which is uses for spell checking.

I remember initialization being extremely sensitive when I implemented multi dictionary support. The timings of when the spell checkers are ready seem to be the hard part to get right. I also think the one-language case will be more reliable, since we can use the lang attribute on the document there to chose the default language the spellcheckers should use on init, however for multiple languages we have to rely on the spellchecker actually remembering the selection.

(In reply to Martin Giger [:freaktechnik] from comment #3)

I remember initialization being extremely sensitive when I implemented multi dictionary support.

Sadly, establishing multiple dictionaries in composition (bug 1766604) also doesn't work 100%. When it doesn't work, the UI looks OK, but spell checking uses a different dictionary. It becomes good when focusing a different window and then returning to the composition. Saving the draft also helps.

Turned out that new dictionaries cannot be set while the spell check is still checking with the initial dictionary. Patch from comment #2 updated.