Ah, I understood the question the other way around, like how to prevent translation.
I don't think the translation of those dynamic (free form) fields is supported. How this is done in other cases is that a login page per language is created, and then use some scripting to redirect users to the right language page.
What might work as well is use the following code in your custom HTML:
{if $current_lang == 'ar'}
PASTE YOUR REDIRECT CODE HERE -- OR WHATEVER LANGUAGE-SPECIFIC CONTENT -- SEE EXAMPLES ABOVE
{elseif $current_lang == 'jp'}
PASTE Japanese here
{else}
Fallback here
{/if}
If you need a more definitive answer, please contact Aruba TAC.