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!