Indeed, but in many countries this is done via a 13th payment.

To be honest I've never seen 13th payments offered in dev careers, so when I hear some of my Scrum friends demanding it I'm quite surprised since I know they make comparable amounts to me.

I'm a dev in Belgium, and I get both my "13th month" in December and double holiday pay in June. In fact, I've never seen anybody here who was traditionally employed (i.e., not contracting through a zelfstandig) who didn't get those perks.

Is it? I've exclusively seen it refer to the extra monthly salary at some point during the year (typically around December). I guess it depends on the country.

12*2 is only twenty four checks. The "13th" month brings you to twenty six checks, which is the actual number of 2 week periods in a year.