1. Locate this file: "<somewhere>\SteamLibrary\steamapps\common\Carrier Command 2\mod_dev_kit\source\locale\localization.csv" and have a look at it in Excel, LibreOffice, or whatever else suits you. You can go to the game folder from the Steam Client via Library | Carrier Command 2 | Gear Icon | Manage | Browse local files.
2. Follow this guide: https://geometa.co.uk/wiki/carrier_command_2/view/modification_development
3. Copy the file from step 1 to the <your_mod_name>\source\locale\ folder, if you haven't already.
4. Edit that file.
5. Delete all other files and folders below the <your mod name>\source\ folder.
6. Update the description in the manifest file.
7. Compile mod.
8. Activate the mod in-game.
9. Test mod.
10. Goto step 4 until satisfied.
11. Upload Mod to Steam Workshop.

I'm interested in creating a language mod too but speak only English and German.

Originally posted by Ludendus:

1. Locate this file: "<somewhere>\SteamLibrary\steamapps\common\Carrier Command 2\mod_dev_kit\source\locale\localization.csv" and have a look at it in Excel, LibreOffice, or whatever else suits you. You can go to the game folder from the Steam Client via Library | Carrier Command 2 | Gear Icon | Manage | Browse local files.
2. Follow this guide: https://geometa.co.uk/wiki/carrier_command_2/view/modification_development
3. Copy the file from step 1 to the <your_mod_name>\source\locale\ folder, if you haven't already.
4. Edit that file.
5. Delete all other files and folders below the <your mod name>\source\ folder.
6. Update the description in the manifest file.
7. Compile mod.
8. Activate the mod in-game.
9. Test mod.
10. Goto step 4 until satisfied.
11. Upload Mod to Steam Workshop.

I'm interested in creating a language mod too but speak only English and German.

Thanks, I already found out the way, but unfortunately some UTF-8 characters are not supported by the font, which I tried to change as well. More over, it's a real pain to edit the localisation file while it's a tabulated CSV. Opening it in OpenOffice or Excel completely breaks the file. I gave up on this, especially beacuse mod support is zero.

Could replacing lanapixel.ttf help?
https://opengameart.org/content/lanapixel-localization-friendly-pixel-font has a selection of options.

I tried with different fonts, even renaming, but even after compiling the game doesn't load it.