Be sure to compile your game with:
-DENABLE_FREETYPE=1 -DENABLE_GETTEXT=1
or use windows build with freetype and gettext support.
Create a folder named after shortcut of your language, then edit .po file inside.
You can use
https://github.com/minetest/minetest/bl ... inetest.po as a template.
Dont forget to set in minetest.conf:
langauge = your_language_shortcut