GUI: fix qt translation load error
Locales are something like en_US / zh_CN / zh_SG.
Before this commit recoll will manually truncate them to en / zh, then load language file 'recoll_en.qm' and 'recoll_zh.qm'. 'recoll_en_US.qm' and 'recoll_zh_CN.qm' will be always ignored.
After this commit, recoll will try to load 'recoll_zh_CN.qm' first. If file is not found, it will fall back to 'recoll_zh.qm'. This is Qt's default behavior.