Set gtk language based on selected language in RT, fixes #4278, kudos to @agriggio

rtgui/multilangmgr.cc

@@ -94,10 +94,34 @@ struct LocaleToLang : private std::map<std::pair<Glib::ustring, Glib::ustring>,
 
         return "default";
     }
+
+    std::string getLocale(const Glib::ustring &language) const
+    {
+        for (auto &p : *this) {
+            if (p.second == language) {
+                std::string ret = p.first.first;
+                if (!p.first.second.empty()) {
+                    ret += "_" + p.first.second;
+                }
+                return ret;
+            }
+        }
+        return "C";
+    }
 };
 
 const LocaleToLang localeToLang;
 
+void setGtkLanguage(const Glib::ustring &language)
+{
+    auto l = localeToLang.getLocale(language);
+#ifdef WIN32
+    putenv(("LANG=" + l).c_str());
+#else
+    setenv("LANG", l.c_str(), true);
+#endif
+}
+
 }
 
 MultiLangMgr langMgr;
@@ -106,8 +130,10 @@ MultiLangMgr::MultiLangMgr ()
 {
 }
 
-void MultiLangMgr::load (const std::vector<Glib::ustring> &fnames)
+void MultiLangMgr::load(const Glib::ustring &language, const std::vector<Glib::ustring> &fnames)
 {
+    setGtkLanguage(language);
+
     translations.clear();
 
     for (const auto& fname : fnames) {

rtgui/multilangmgr.h

@@ -30,7 +30,7 @@ class MultiLangMgr
 public:
     MultiLangMgr ();
 
-    void load(const std::vector<Glib::ustring> &fnames);
+    void load(const Glib::ustring &language, const std::vector<Glib::ustring> &fnames);
     Glib::ustring getStr(const std::string& key) const;
     static bool isOSLanguageDetectSupported();
     static Glib::ustring getOSUserLanguage();

rtgui/options.cc

@@ -2323,7 +2323,7 @@ void Options::load (bool lightweight)
         }
     }
 
-    langMgr.load ({localeTranslation, languageTranslation, defaultTranslation});
+    langMgr.load (options.language, {localeTranslation, languageTranslation, defaultTranslation});
 
     rtengine::init (&options.rtSettings, argv0, rtdir, !lightweight);
 }