... and ignore language setting (e.g. en_US).
The problem came from the fact that these settings names are class
properties of GimpPaintOptions/GimpContext which is first instanciated
when the Gimp object is created. This unfortunately happened before
language_init() since we needed these objects when loading gimprc
(making inversion of calls rather complicated).
Therefore they were localized with the system language, not the
configured language.
The solution was to create a very simple object GimpLangRC which
implements the GimpConfig interface, for sole purpose to read the
language from `gimprc` in a first pass. gimp_load_config() will still
happen later as a second pass to properly load the rest of the
configuration.