Per-project notifications + same-language only

### Describe the problem

SAME LANGUAGE:
As a translator or a reviewer (not maintainer), you are usually interested to receive only comment notifications from users working on same language. This did not happen. Notification i have received was for comments made by two users, which did not have my language set in their profile.

ONLY CERTAIN PROJECT(S)/EXCLUDE OTHERS
I would like to receive comment notifications only for a project(s) where i am a proofreader/reviewer, so i have power to do something about possible issue. It is waste of my time and server resources to be notified for a comments in every project, where i am mere translator. Also i may just be interested to receive notifications for one or two projects, not all.
I do not see easy way to configure this at https://hosted.weblate.org/accounts/profile/#notifications
There are 3 categories of notifications based on a project type: watched, managed, other.
Maybe it would help if there is a custom category of a notifications where i can assign projects of a choice, similarly as done for Watched projects on a /accounts/profile/#notifications ?

CURRENT STATE:
Makes me disable comment notifications entirely for all projects to avoid different language comments which i often can not address/not interested in or comments from projects i do not want to deal with in depth.

### Solution brainstorm

- I would expect by default to receive comment notifications only for my language.
- I would expect to have groups of notifications where i can exclude certain project by simple click (similar to Watched projects section at /accounts/profile/#notifications)

The stated entry point is /accounts/profile/#notifications; start by tracing how comment notifications are grouped into watched, managed, and other projects. Done means users can restrict comments to their language and include or exclude selected projects without disabling comment notifications globally.

Written by the indexing model from the issue text.