Meow

Game Skyrim Special Edition.
Can I export journal to TXT?

There's probably a mod for this, but what are you seeking to accomplish?

To see, which quests are already done.
Maybe to sort list in alf abetic. Then coluld be easier to find which uest is already done.

I am confused. Doesnt Skyrim show you which quest is already completed in the journal? Also do you need to export just the names of the quest? I don't know how to export them but in general if you have them on screen in the journal you can try use a program like PowerToys. With it you can use the Text Extactor component and extract(copy) any text on your screen and paste it in text editor. Not sure if it's going to recognise the text cause the game is very dark but you can try!

Hypothetically, but boy does Bethesda Software just work.

Yes.
OP could simply write down his/her list of finished/unfinished quests, manually - though I fail to see the point behind that effort.