add support for en.json translation files (#976)

docs/release-notes.md

@@ -6,6 +6,9 @@
   * Improved content load performance for non-English players.
   * Fixed some community shortcuts breaking if a mod edited the map which contains them.
 
+* For mod authors:
+  * You can now have an `en.json` translation file which overrides `default.json`.
+
 * For the web UI:
   * Improved JSON parser:
     * You can now hover/click braces to highlight matching pairs.

src/SMAPI/Framework/Translator.cs

@@ -165,6 +165,10 @@ internal class Translator
     /// <param name="locale">The locale for which to find valid locales.</param>
     private IEnumerable<string> GetRelevantLocales(string locale)
     {
+        // special case: the game's locale code for English is an empty string, but we still want to allow 'en' as a locale
+        if (locale == string.Empty)
+            locale = "en";
+
         // given locale
         yield return locale;