User should recive mails in the account preferred language

reviewed, made a change, if the user preferred language is not set then use language_default()

got a little more timid on this change,
message language, otherwise user preferred language and if not defined, then site default language

I am getting an error after installing the 7.x-1.20 version:

( ! ) Fatal error: Cannot use isset() on the result of a function call (you can use "null !== func()" instead) in /var/www/rovdev/sites/all/modules/i18n/i18n_user/i18n_user.module on line 13

I fixed it by creating a variable for the user_preferred_language($message['params']['account']) call and then changed the isset to test that variable. I can produce a patch, but it will take me a while as I am a noob at that. Let me know if interested.

I got the same error when updating on 1.20.

Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead) in ... /i18n/i18n_user/i18n_user.module on line 13

Is the suggestion in the error message a solution?

 if ($message['module'] == 'user') {
    $user_preferred = language_default();
    if (null !== user_preferred_language($message['params']['account'])) {
      $user_preferred = user_preferred_language($message['params']['account']);
    }

yes sorry, fix comming shortly.

fixed in release 7.x-1.22 , I put an extra check in the dev branch for next release but should work either way.

fixed in 7.x-1.x dev branch