Not a native, but from what I saw in the comments, 'rarely' definitely wins.
Definitely (native here, as you know). Unless you're talking about which was more common in 1800, when seldom won by a landslide. Your useless fact for the day. :)
These days seldom is less common than rarely and has a stronger, more emphatic nuance. "They seldom buy cheese" would usually imply that this fact is a big deal.
"seldom" is still used by educated English-speaking people. I would suggest that "rarely" is stronger than "seldom".
I used "rarely" because "rzadko" starts with R.
"purchase" is another word for buy. "They seldom purchase cheese."