During the re-release we did change the folder structure cater better for multiple translations - so there are <language> subfolders - and assets that does not need translations are not redundant all over the place. So the "Launcher" will populate the "language" selector by the "folders" found. So yes - it is fairly easy to add new languages without any changes to the source code. Unless there is some fun - with character codepoints and encoding - like we had when the PL and RU versions surfaced
The text should be complete in some form, as an example are the:
Message109="Note: Actually not designed for a female character"
Message110="Do you want to play the tutorial?"
Message111="In Hardmode there's no possibility to save your game on your own!"
which are missing in the Czech Text.ini - since they are newer that the Anthology re-release, which means that dialog is blank or not showing. Just throwing the English in is a option