The English translation of the Basic Law (a document that is for all intents and purposes an long term interim constitution) states in Section 5, Article 54, Clause 2 (emphasis mine):
The term of office of the Federal President shall be five
years. Re-election for a consecutive term shall be permitted
This wording appears to state that the President of Germany can be elected to an unlimited number of terms but can not run for a third consecutive term (i.e. they must sit out for a term before being able to be elected again).
Link to the English translation in PDF form
English translation from the German Cabinet's website