To change the Browsers standard language open the menu > options > general > Language > Search for more languages... > Select a language to add

(see added images).

If this didn't solve your problem, pleas let us know.

Hi KP Bucher, if you want a translator for use in web pages, you could consider this Recommended add-on, or other translation add-on:

https://addons.mozilla.org/firefox/addon/to-google-translate/

Hello kpbucher,

Just in case you're not all that keen to use an extension;   maybe you'd like to give this a try :

https://translate.google.com