Doesn't load localised description of skyculture/landscape if applanguage=system_default

[375gnu]

If we have have `app_locale' in config.ini set to `system_default' then real locale name is `xx_XX' and in such form it's returned by getAppLanguage().
When stellarium looks for translation for sky culture or landscape it looks for file `description.$lang.utf8'. Of cause it couldn't find it cause it expects $lang to be `xx' not `xx_XX'.

I think it should check both files.