My systems have

  LANG=de_DE.UTF-8
  LC_MESSAGES=C

There is substantial disagreement among the programs that I use [1] whether this constitutes a German locale or an English locale.

[1] True, only those programs which are dumb enough to assume that locale is described by a single word like "German" or "English".

Getting system languages isn’t straightforward nor does it indicate the actual language of the user same goes for getting the language from the website. Not to mention that is unclear if you do that does it count as personal information or not since there are some wild cards in the GDPR.