As simple as 1, 2, 3... !
- Go to your admincp, in Appearance > Languages
- Click on the Import a Language button
- On the opening page:
- Select your file from the file explorer
- Select the "Overwrite language" option
- Select the language you want to overwrite (don't worry it will only overwrite the items relative to the add-on)
- Click on Import
- And voilà !