Reference formulas in non-English text books In a mathematical paper, is it wise to refer to formulas in a non-English (here: German) text book, under the conditions that:


the text book is standard in its language region
all English text books lack the desired presentation 


In my case, I am concerend about some formulas from vector analysis, which not too difficult, but American text books do not seem to feature these formulas.
You should cite the most appropriate references for your work, regardless of the language of the source. (I suspect that even mathematics, many of the most important references were initially published in other languages, such as French, German, or Russian.) 

You are not required to cite only English-language sources in publications, especially if the only source you can find is in another language. However, you may have to find a way to share the reference with editors or referees, particularly if the reference is rather obscure. If it's published by a mainstream company, however, then it should not be a problem to verify it. (I should point out that the importance of this depends critically on how central the reference is; a minor result that can be independently verified is not a problem; a core result requires much greater scrutiny.)