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.

"13th payment" is the same as getting paid every two weeks (quite common), but concentrated onto one moment instead of evenly paced.

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.