Translation:The seal, which lives close to the wadden islands, belongs to this family of animals.
My translation was "The seal that lives close to the wadden islands belongs to this family of animals." It was counted wrong and a bubble popped up explaining that "that is only used in restrictive clauses, and the commas imply the use of which" or something close to that effect.
There was a similar translation below that was word-for-word the same as my answer, except the word "to" was omitted from the phrase "close to." I'm unsure of whether to report my answer as correct as a result, but I'm going to anyway.
In any case, making a distinction between "that" and "which" is rather unnecessary and nit-picky, especially since we assume that the learner knows good English. In common English, I think that they would be used almost interchangably in this case. If I spoke this or a similar sentence aloud, I don't know if I would ever choose "which" no matter the nuances between the two.