as far as I know, there no special requirements for the relays in cross-country skiing at WCH and WOG. only the age limit, the youngest allowed skier can not be younger than 16 years. so in fact every country which has qualified at least 4 skiers for the WCh or WOG is free to enter a relay.
Too bad that this win means nothing for 2018 Olympics as the 15km will be in free where Niskanen is not so good...
How can swimming have four different styles while XC skiing can't even have two for the interval starts at least...
I'm currently in Galway, Ireland, as an Erasmus student and today I went to the seaside and took a lot of pictures (more than just the ones I'm putting here):
I'm sorry for posting them like that but it was the only way I found (I tried other ways but it didn't work...)