Are you able to get the lining paper all off? That would be very good news. Are the walls smooth enough; few gouges? More good news if so.
If the walls are in decent condition, you could just prime and paint. You'll probably need a few coats of primer. It may not be glass-smooth, but that can be a good look.
If you have bits where the paper won't come off, lots of gouges, or if the walls are crumbly-dry... you'll want to re-finish the whole thing. You'll have a few options. The most traditional will be to re-plaster. If you want perfection, call a pro.
If available here... I re-finished crumbly, damaged walls by sealing with a kind of PVA. The type that remains sticky forever. On top of that, I mixed up a product made of dead plankton. It mixed into a thick, papier mache-like porridge. I put it on the walls thickly with a flexible trowel... and all the gouges were filled. It dries quickly, and comes in a few colours. It can be painted. Not a solution if you're going for a very smooth look... and not sure what it's called in English. Easier than re-plastering, imo.
Maybe a plaster-master will chime in and tell us how easy it is, though! Small bits I can do, whole walls... not so good.