Just answering my own question :)
I solved it by passing an Ampscript variable, setting it up as a hidden field on form submit, and calling it during Post passing the subscriber's language to the data extension.
Something like this:
Sign-up page:
(Ampscript)
%%[ set @formlang = "en-CA" set @formlang = "fr-CA" ]%%
*(HTML) *
<input type="hidden" name="formlang" id="formlang" value="%%=v(@formlang)=%%"/>
Confirmation page - on POST
(Ampscript)
%%[ set @formlang = RequestParameter("formlang"); UpsertData ('lang', @formlang) ]%%
if you have an alternative to the above, share it here - thanks!