Bug #16162 highlighted the lack of regression tests run under different locales. Running the whole test suite (occasionally, if not after every commit) under different locales would help catch regressions in locale-dependent parsing, string output etc. There is probably no need to write new tests. One need only modify the test harness to re-run the suite with a variety of LC_ALL values set for issues like this to be spotted.
Good idea! It may actually be enough to set any locale that is sufficiently different from "C" one - maybe a simple setlocale(LC_ALL, "fr_FR") call would do in practice.
See also: bug 18994.
And bug 18985.
Is this really Gtk-specific? I am removing the keyword, as I think not.