If I have a website and want to offer options for translation, should the option for translating it to German be “Auf deutsch” or “In deutsch”? Most websites just say Deutsch or German, but our convention is “In English”, “En Español”, etc.
There is a rather long answer in German in the related question. It boils down to the fact that both versions are possible. However, in certain contexts one of the two feels more natural. In your context of a translated website, I would prefer:
Corresponding is the usage when asking for translations:
Was ist denn translation auf Deutsch?
Or when writing something oneself:
Er schreibt einen Brief auf Deutsch.
(in is possible, however, auf sounds more natural to me.)
If we are no longer creating, or if we wish to add an adjective, in is usually the better choice:
Hier sind Beispiele in Deutsch und Englisch.
Ich möchte das in gutes Deutsch übersetzen.
And likewise, often an in variant can be made more natural by adding an article, thereby impyling direction:
Wie sagt man das im Deutschen?
Ich übersetze Ihren Brief ins Deutsche.
And when referring to the school subject, use of in is mandatory:
Ich hab einen Vierer in Deutsch bekommen.
You can subsitute Deutsch for any other language throughout this answer.
If you liked this answer, please also consider upvoting OregonGhost’s answer to the German related question from which I strongly borrowed.