Hi Alice
I assume you mean non-English language, rather than non-British? Because the first non-British would of course be the 1905 Broadway production in New York. As far as non-English language is concerned, I have only ever come across a reference to the 1908 French production as the earliest, but I have no other details.
With respect to a German production, I don't have any information on this, but the theatre agents Felix Bloch Erben (www.felix-bloch-erben.de) may be able to help, as they handled the performing rights of Peter Pan for GOSH for many years (until 2007, when it entered the public domain) and they may have a record of the history of the German version. If you find out, do let us know!
Good luck.