If a theme is translatable, it’s very easy to add more languages by just copying the 
English file en.json as e.g. 
Finnish file fi.json (or other ISO language code), and translating the right side of each line.
For example, for WorldCasper2, you can see the English file en.json, with 25 half lines to translate.
Some translatable themes, in alphabetical order:
Canitia Jr.
https://canitia.nl/themes.html#canitiajr
https://github.com/Canitia/canitia-jr
Clamo
Ghost Blank Theme
https://github.com/rogeriorioli/ghost_blank_theme
Ghost Casper Theme Turkish Translation
Ghost Theme Template
Mapache
Simply
It’s used here:
UnivUnix
WorldCasper1
WorldCasper2