I think that the code to change is in js/ui/calendar.js:
        // nRows here means 6 weeks + one header + one navbar
        let nRows = 8;
        while (row < 8) {
            let button = new St.Button({ label: iter.getDate().toString(),
                                         can_focus: true });

We probably just need to replace the "toString" with something that's translatable, correct?

(In reply to comment #1)
>             let button = new St.Button({ label: iter.getDate().toString(),
>                                          can_focus: true });
> 
> We probably just need to replace the "toString" with something that's
> translatable, correct?

Yes, this sounds right. Something like
  iter.toLocaleFormat(_("%d"))
should do the job.

Created attachment 309948 [details] [review]
calendar: Fix number formatting in some languages

Some languages require using a conversion modifier to display the day
number in a translated shape. fa_IR is affected for example.

Review of attachment 309948 [details] [review]:

LG.

Attachment 309948 [details] pushed as 3e602b1 - calendar: Fix number formatting in some languages