fix(i18n): match two__letters code to crowdin (#2289)

docs/source/about/advanced_usage.rst

@@ -63,8 +63,8 @@ editing the `conf` file in a text editor. Use the examples as reference.
    =======   ===========
    de        German
    en        English
-   en-GB     English (UK)
+   en_GB     English (UK)
-   en-US     English (United States)
+   en_US     English (United States)
    es        Spanish
    fr        French
    it        Italian

src/config.cpp

@@ -1105,8 +1105,8 @@ namespace config {
     string_restricted_f(vars, "locale", config::sunshine.locale, {
                                                                    "de"sv,  // German
                                                                    "en"sv,  // English
-                                                                   "en-GB"sv,  // English (UK)
+                                                                   "en_GB"sv,  // English (UK)
-                                                                   "en-US"sv,  // English (US)
+                                                                   "en_US"sv,  // English (US)
                                                                    "es"sv,  // Spanish
                                                                    "fr"sv,  // French
                                                                    "it"sv,  // Italian

src_assets/common/assets/web/config.html

@@ -43,8 +43,8 @@
           <select id="locale" class="form-select" v-model="config.locale">
             <option value="de">Deutsch (German)</option>
             <option value="en">English</option>
-            <option value="en-GB">English, UK</option>
+            <option value="en_GB">English, UK</option>
-            <option value="en-US">English, US</option>
+            <option value="en_US">English, US</option>
             <option value="es">Español (Spanish)</option>
             <option value="fr">Français (French)</option>
             <option value="it">Italiano (Italian)</option>