FB has a setting for language. Click on the icon which has your profile image then > Settings & Privacy (the first option with >) > Language (the last in the list which you then get). Does that fix the problem? You may need to get Dutch translations for those.

What Firefox locale do you use in case this makes a difference ?

• https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages

Some website specify the locale as a parameter in the link, so make sure you do not use a link (bookmark) that specifies Dutch like "nl" or "nl-nl".