Didn't know that! Can you upload custom dictionaries to them? That was my only gripe with how Kindles do it - they support this feature in some languages but not others.
The dictionary is a built-in official feature and includes options in few different languages but CUSTOM dictionaries are not. It's possible to add dictionaries beyond what Kobo includes but it takes a little bit of work.
It seems to be a limited selection of languages (EN, DE, ES, FR, IT, PT, TR, SE, JP on my Kobo), with both EN-XX and XX-XX available for most languages. It doesn't look like you can add your own, at least not on my kobo.