Which language for which job? [on hold]

Which language for which job? [on hold]

Today I asked myself a quite fundamental question, well .. I guess it is
one. But google- and SEing couldn't give me the answer I was looking for.
I'm writing programs for quite a few years now, but the languages in which
they were written, was always given by higher seniors, customers or by
myself for learning purpose.
Let's say for instance that I want to write a application with basic
functions, like database access, calling webservices and so on.
Which language should be used, and why? Ignoring factors like
cross-platforming, UI, which DB system or webservices are used ... imagine
something like a black body in physics.
As example, why should I use Java over C++ or Python over Ruby? Not saying
that this question is limited to these languages.
Are there good reasons for choosing an specific language, like performance
or memory usage, or is it, in case of a "black-body-situation", just the
personal preference which determines the actual used language?
Thanks in advance.
... and sorry if this question is to general.