Not really an answer, but a suggestion on how to go about finding an answer while keeping a modicum of objectivity.
Although the vast majority of papers on international journals are in English, some of the journals accept papers in other languages. Check the top journals in your favourite area and see which languages they accept. Most of the time it's only French and German. This may be historical or, as in the case of Springer journals, possibly geographical.
In my experience, I've found French to be the most useful language after English for contemporary papers and also, together with German, for older (say, late 19th, early 20th century) papers. Older than that and I find that there's a problem of mathematical language, regardless the natural language into which the mathematics happens to be embedded.
Alas, my mother tongue (Spanish) has very little mathematical literature :(
I noticed the CW suggestion after I had started typing the answer. So to restate, French has proved the most useful to me, and the topics are Differential Geometry and to some extent Lie theory. A close second is German, on the same topics. My main research topic, though, is Mathematical Physics and this seems to be almost all in English.