Disclaimer: I still use The Bat v8.8.2.

I can choose Lithuanian in the spell checker of the message editor. However, I don't know if that was always possible, or that it is because I just added the dictionary myself..

What I did is this:

In the Speller subdirectory of the program folder, I found a lot of .DIC and .AFF files for various languages, but not for Lithuanian. I went to look online for lt_LT.aff and lt_LT.dic. I found them here:

https://github.com/ONLYOFFICE/dictionaries/blob/master/lt_LT/lt_LT.aff  (I right-clicked on 'raw' and used Save Link As)
https://github.com/ONLYOFFICE/dictionaries/blob/master/lt_LT/lt_LT.dic  (I right-clicked on 'download' and used Save Link As)

I saved both files in the Speller directory.

When I launched The Bat and went to Options | Language, I did not see Lithuanian in the list.

However, when I created a new message, and in the editor I chose Spell Checker | Languages, I could pick 'Lithuanian' there..!

So, maybe you should first look in the spell checker of the message editor if Lithuanian isn't already present there. If not, you can do what I did and hopefully that will add the Lithuanian language to the editor for you.

Note: In the Spell Checker menu of the editor, there is also a Dictionaries option where it seems you can import and export dictionaries. Lithuanian is mentioned there, too, but I have no idea how it works.