Hi!
Something like de+Q1980305 could work; when generating HTML or RDF, we'd just drop the suffix. For transligual entries (e.g. the for number symbol i), we could use e.g. mis+Q1140046.
I think for those that are not in particular language, und or zxx could be better. mis as I read it is for "this is in specific language, but we don't have a code for it". See https://en.wikipedia.org/wiki/ISO_639