change setValue to replace value instead of insert

CHANGELOG.md

@@ -6,6 +6,7 @@
     Bug #5379: Wandering NPCs falling through cantons
     Bug #5453: Magic effect VFX are offset for creatures
     Bug #5483: AutoCalc flag is not used to calculate spells cost
+    Bug #6037: Morrowind Content Language Cannot be Set to English in OpenMW Launcher
     Bug #6066: addtopic "return" does not work from within script. No errors thrown
     Bug #6067: esp loader fails in for certain subrecord orders
     Bug #6101: Disarming trapped unlocked owned objects isn't considered a crime

components/config/settingsbase.hpp

@@ -24,9 +24,7 @@ namespace Config
 
         inline void setValue(const QString &key, const QString &value)
         {
-            QStringList values = mSettings.values(key);
-            if (!values.contains(value))
-                mSettings.insert(key, value);
+            mSettings.replace(key, value);
         }
 
         inline void setMultiValue(const QString &key, const QString &value)