Not sure if you solved this, but I would approach this with MULTIPLE guest self-reg pages, with one language each, with each having a separate "email template" per its own language.
Create a splash page to detect the user's default browser language and redirect accordingly:
{if $current_lang == 'ar'}
PASTE YOUR ARABIC GSR REDIRECT CODE HERE
{elseif $current_lang == 'fr'}
PASTE YOUR FRENCH GSR REDIRECT CODE HERE
{else}
PASTE YOUR DEFAULT/CATCH-ALL/ENGLISH GSR REDIRECT CODE HERE
{/if}
And for reference, here are the language codes:
ar, da, de, en, es, fr, ja, ko, nl, zh
An example of a redirect code:
<meta http-equiv="refresh" content="1;url=http://google.com"/>
And of course create each unique email receipt template:
CPGuest > Config > Receipts > Templates > (duplicate the one you want and create one per language)
... And lastly, make sure each guest self-reg page references its respective language email receipt template!