The hyphenation algorithm, I guess, needs some knowledge of the language. Probably a lot more issues as well.
Both fit neatly into the same codepage, latin-1, so no possible conflicts there
> I place \selectlanguage{UKenglish} and \selectlanguage{ngerman} in front of the appropriate paragraphs
What about mixing two languages within the same paragraph? \selectlanguage before and after each word?