"Lektionen är svenska."

Translation: The lesson is Swedish.

[deactivated user]

Does this refer to a Swedish language class or any lesson taught in Swedish?

[Lundgren8]

It refers to a class teaching Swedish, so it is a noun here just like the name of other subjects such as ”history”, or ”geography”. If it were an adjective describing the lesson, it would be ”svensk”.

[deactivated user]

Gotcha. Thanks!