The intersection of two planes is always a line?

Assuming that they do intersect but are not coincident, then Yes.

Just to make this a little simpler, since if you're asking this, you may not understand the above answer. As long as they're not parallel, e.g., like two floors in the same building are parallel and will never meet no matter how long you extend them, then they'll meet eventually, and will form a line. Coincident just means they are in exactly the same space, and therefore are essentially identical and only one plane, and so also will not intersect.