Room interior wall not built straight

Compensation, if there is any, will be very low. The "damage" is so large that it is noticed by chance. Presumably, the cost of the process to possibly receive compensation is many times the amount of the compensation. The tolerances should be listed in DIN18202

Can someone tell me what would be an appropriate compensation here.

None. If you insist on a right-angled room, have it corrected. If it is not that important, then no damage has been done. It does not affect the value of the house.

What was given as the reason for the slanted wall? It could quite well be "normal" if you are renovating a house from 1850.

The room is narrower..

And the room on the other side of the interior wall is therefore wider. So two skewed rooms?

You have already accepted the house. I am not sure, but the defect should have been reported in time, since apparently the defect is absolutely not obviously disturbing. It was only noticed during the installation. Complain about it in writing and in proper form. I don't think you will get more than the amount you have to add to the floor covering, that is about €150 -> material plus an hour of work. If the contractor cooperates. If he does not acknowledge it, then you get nothing, and a lawsuit is not proportionate anyway.

10cm [...] The total length of the wall is 3.54m.

It's just annoying when you wanted to place something right-angled in the corner. A 2-meter bed then stands about 6 cm away from the wall. And that affects two rooms.

But after acceptance? Well ...

I’m also not quite sure how this can happen. Connection to the exterior wall marked wrong and the bricklayer just stubbornly carried on? It should be noticeable (before plastering!) if the bricks don’t fit exactly together.

best regards