The system Spellchecker cannot recognize Lithuanian Language despite it being in the list

On High Sierra my default Apple Spell-checker cannot recognize some languages, like Lithuanian. It affects mostly my email composing, because I use Apple Mail, which doesn't have it's own spellchecker. The spellchecker is set to Automatic by Language. English and some other languages are Ok, but not Lithuanian. I've read some people have the same problem with other East European languages. If I open the Spelling tab and change the Dictionarry to Lithuanian (this dictionary did not exist by default on Mac, and I've installed it separately a while ago), it still shows everything underlined in red, as if it cannot recognize the language. But If I go word by word in the Spellchecking window, it does start to recognize it.