> I also need to be sure that a language (e.g. GSL) [...]
Like others I'm delighted to hear of your interest in GSL, but I do need to point out that GSL is *not* a language -- it's a library of routines that can be called from various languages. The most natural is C, but most widely used VHLLs have GSL bindings. GSL's first release was in 1996, and it continues to have solid use, so you don't need to worry about longevity there. And the longevity of C code is extreme: the C standards committee is very careful about not disrupting the possible use of old code. As an example, I recently rebuilt GSL code from 20 years ago and it required almost no effort.
