It looks to me like you can do it by following these steps:

1. Click the menu button in the upper right (three horizontal lines)
2. Click on the "settings" option
3. In the "search" bar within the settings menu, type "Language"
4. For the top result, click the dropdown menu (see attached)
5. Click "Search for more languages..."
6. In the popup settings panel, click "Select language to add..."
7. Select German from the list
8. click the "Add" button
9. If prompted, confirm the action.
10. Click "Ok"

You should now be able to set Firefox to use German or English, from this selection.

(There is likely a simpler way to do this, with which I am not familiar)

How do I change language to german?

The language of the Firefox user interface like the menus and such or are you referring to the websites?

https://support.mozilla.org/en-US/kb/use-firefox-another-language https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages

Or if you prefer to download Firefox in German language then go to https://www.mozilla.org/firefox/all/#product-desktop-release and use the Select your preferred language drop menu for German.