Windows and Mac implementations return a string without any encoding
specifier -> remove this specifier directly in the Unix/Linux
implementation for consistency.
Also do some small refactoring with the new static method
FGLocale::removeEncodingPart(). Slight difference with the previous
algorithm: if a '.' is found in the given locale spec, we assert() that
it is not the first character. The previous code in
FGLocale::findLocaleNode() used to consider such weird locale specs
starting with a dot as normal locale specs without any encoding part.
Note: the same change could be done where FGLocale::findLocaleNode()
looks for an underscore in order to prepare for the fallback
search (e.g., 'fr' after not finding translations for 'fr_FR').
Respect the user's language selection in the system settings, and try those languages in turn. If a language is explicitly specified, try that first regardless.
