Select "English" if the current language is not found in Preferences

If the user preferences file (aseprite.ini) contained a non-existent
language, the first option of the languages combo box was selected,
which might lead to a confusing situation where just opening the
preferences dialog will change from English to other
language (non-English, the first language in the combobox).

src/app/commands/cmd_options.cpp

@@ -1317,9 +1317,20 @@ private:
     if (language()->getItemCount() > 0)
       return;
 
-    // Select current language by lang ID
+    // Check if the current language exists, in other case select English.
     Strings* strings = Strings::instance();
     std::string curLang = strings->currentLanguage();
+    bool found = false;
+    for (const LangInfo& lang : strings->availableLanguages()) {
+      if (lang.id == curLang) {
+        found = true;
+        break;
+      }
+    }
+    if (!found)
+      curLang = Strings::kDefLanguage;
+
+    // Select current language by lang ID
     for (const LangInfo& lang : strings->availableLanguages()) {
       int i = language()->addItem(new LangItem(lang));
       if (lang.id == curLang)

src/app/i18n/strings.cpp

@@ -25,7 +25,8 @@
 namespace app {
 
 static Strings* singleton = nullptr;
-static const char* kDefLanguage = "en";
+
+const char* Strings::kDefLanguage = "en";
 
 // static
 void Strings::createInstance(Preferences& pref,

src/app/i18n/strings.h

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2023  Igara Studio S.A.
+// Copyright (C) 2023-2024  Igara Studio S.A.
 // Copyright (C) 2016-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -25,6 +25,8 @@ namespace app {
   // Singleton class to load and access "strings/en.ini" file.
   class Strings : public app::gen::Strings<app::Strings> {
   public:
+    static const char* kDefLanguage;
+
     static void createInstance(Preferences& pref,
                                Extensions& exts);
     static Strings* instance();