Multiverse

Connections:
There are collections of realms that have non-Rift connections between them. The nine Realms of Norse Mythology are all interconnected by the Bifrost which the Norse Gods created. Rift hates the thing, as it is nowhere near as elegant as her system, but grudgingly allows it to exist, as Asgard is on her side in the Plainswalker/Shadow war.

Languages:
Languages across the realms are mostly the same. English is one of the most common, as are other ones used on earth, like Japanese. An example of why this is can be found in the dragon language. All dragons were originally created, or at least, most of them were, by Rift and Void, so all of them would have the same language due to the Primordials being lazy, and not wanting to create a new language for each realm. However, due to the differences from realm to realm, each realm has its own 'accent' in the dragon language.

This means that if Toothless and Ancalagon the Black met, they could understand each other, but Toothless would sound Scandinavian, and Ancalagon would sound like he was speaking Old English. However, if Toothless met a Dragonite, their language systems wouldn’t be compatible, as their species- or the ancestors of them- were created by different primordials.