1. Install language packs: use MX Package Installer > Language, and select and install what you need
2. Add the add-on: a small colored notice will appear in the upper right corner. Click on it and then click to add.
3. Enabling. Firefox 59 on:
- Type about:config in the addres bar
- Create a new string with this name:
- Then two choices:
. Leave the intl.locale.requested as an empty-string. When you next launch Firefox, the language will be selected as specified by the LANG environment,i.e. your default system langauge.
. Finish it with the appropriate value (fr-FR, es-ES,…)