Fix setting english language in db_old, document how not setting defaultlanguage really works

data/example.conf

@@ -541,7 +541,7 @@ options
 
 	/*
 	 * Default language that non- and newly-registered nicks will receive messages in.
-	 * Leave empty to default to English.
+	 * Set to "en" to enable English. Defaults to the language the system uses.
 	 */
 	#defaultlanguage = "es_ES.UTF-8"
 }

modules/database/db_old.cpp

@@ -539,7 +539,7 @@ static void LoadNicks()
 				case LANG_JA_EUC:
 				case LANG_JA_SJIS: // these seem to be unused
 				default:
-					nc->language = "en_US";
+					nc->language = "en";
 			}
 
 			READ(read_uint16(&u16, f));