PHPMailer::setLanguage(): make return value consistent

The return value of the method was inconsistent as it could return `true` even when the requested language was not loaded/set.

Previously, the method would:
* Return `false` when a `$langcode` was matched against the regex, but the file couldn't be loaded.
* Return `true` in all other cases, including when a `$langcode` other than `'en'` was passed, but it didn't match the regex, which meant that effectively the requested language was ignored and English was loaded anyway, but the function would still return `true`.

This has now been changed to:
* Return `true` when the requested language was loaded, whether `'en'` or another language.
* Return `false` when the requested language wasn't loaded and the language defaulted to English.

Related to PHPMailer#2418 (comment)

src/PHPMailer.php

@@ -2186,14 +2186,13 @@ public function smtpClose()
 
     /**
      * Set the language for error messages.
-     * Returns false if it cannot load the language file.
      * The default language is English.
      *
      * @param string $langcode  ISO 639-1 2-character language code (e.g. French is "fr")
      * @param string $lang_path Path to the language file directory, with trailing separator (slash).D
      *                          Do not set this from user input!
      *
-     * @return bool
+     * @return bool Returns true if the requested language was loaded, false otherwise.
      */
     public function setLanguage($langcode = 'en', $lang_path = '')
     {
@@ -2248,11 +2247,14 @@ public function setLanguage($langcode = 'en', $lang_path = '')
             //Calculate an absolute path so it can work if CWD is not here
             $lang_path = dirname(__DIR__) . DIRECTORY_SEPARATOR . 'language' . DIRECTORY_SEPARATOR;
         }
+
         //Validate $langcode
-        if (!preg_match('/^[a-z]{2}(?:_[a-zA-Z]{2})?$/', $langcode)) {
+        $foundlang = true;
+        if (!preg_match('/^[a-z]{2}(?:_[a-zA-Z]{2})?$/', $langcode) && $langcode !== 'en') {
+            $foundlang = false;
             $langcode = 'en';
         }
-        $foundlang = true;
+
         $lang_file = $lang_path . 'phpmailer.lang-' . $langcode . '.php';
         //There is no English translation file
         if ('en' !== $langcode) {