Log the message "GMST ... not found" only once for each missing l10n GMST

apps/openmw/engine.cpp

@@ -778,16 +778,21 @@ void OMW::Engine::prepareEngine()
     mEnvironment.setWorldScene(mWorld->getWorldScene());
 
     const MWWorld::Store<ESM::GameSetting>* gmst = &mWorld->getStore().get<ESM::GameSetting>();
-    mL10nManager->setGmstLoader([gmst](std::string_view gmstName) {
-        const ESM::GameSetting* res = gmst->search(gmstName);
-        if (res && res->mValue.getType() == ESM::VT_String)
-            return res->mValue.getString();
-        else
-        {
-            Log(Debug::Error) << "GMST " << gmstName << " not found";
-            return std::string("GMST:") + std::string(gmstName);
-        }
-    });
+    mL10nManager->setGmstLoader(
+        [gmst, misses = std::set<std::string, std::less<>>()](std::string_view gmstName) mutable {
+            const ESM::GameSetting* res = gmst->search(gmstName);
+            if (res && res->mValue.getType() == ESM::VT_String)
+                return res->mValue.getString();
+            else
+            {
+                if (misses.count(gmstName) == 0)
+                {
+                    misses.emplace(gmstName);
+                    Log(Debug::Error) << "GMST " << gmstName << " not found";
+                }
+                return std::string("GMST:") + std::string(gmstName);
+            }
+        });
 
     mWindowManager->setStore(mWorld->getStore());
     mWindowManager->initUI();