Add language/country code comment for RECAPTCHA_LANG

The RECAPTCHA_LANG key in the language packs (captcha_recaptcha.php) might be wrong in some of the language packs. Like around 20. See https://www.phpbb.com/community/viewtopic.php?f=566&t=2468051

So perhaps it would be a good idea to add a comment in the files with link to a page where translators can find the correct value for their translation so there can be less incorrect values in the future.

Like:

    'RECAPTCHA_LANG'                => 'en', // Find the language/country code on https://developers.google.com/recaptcha/docs/language - If no code exists for your language you can use "en" or leave the string empty...{{}}

I'm not sure what the value should be if there is no language/country code for the translation.

But i comment could improve this.