Your deletion of [[Module:accel/ro]]

For that, you have to edit the template itself so that includes the information needed to make the acceleration work.

Can you give me some pointers as to how I can achieve that? Is there a guide? When I look through Module:de-noun which powers {{de-decl-noun-n}} and the likes for which acceleration works I don't see anything relating to acceleration.

Module:nl-adjectives#L-275 shows how the acceleration is added.