- Fix multiple translations in a same process for different id_lang

If id_lang are mixed (1, 2, 3, 1, 4, 2...) the translations aren't correctly populated from the theme lang.php override if include_once is used.
include must be used in order to handle mixed id_lang in a same process.

Signed-off-by: hadjedjvincent <v.hadjedj@presta-module.com>

classes/Mail.php

@@ -372,11 +372,11 @@ class MailCore
 
 		$file_core = _PS_ROOT_DIR_.'/mails/'.$iso_code.'/lang.php';
 		if (Tools::file_exists_cache($file_core) && empty($_LANGMAIL))
-			include_once($file_core);
+			include($file_core);
 
 		$file_theme = _PS_THEME_DIR_.'mails/'.$iso_code.'/lang.php';
 		if (Tools::file_exists_cache($file_theme))
-			include_once($file_theme);
+			include($file_theme);
 
 		if (!is_array($_LANGMAIL))
 			return (str_replace('"', '"', $string));