Enable source string notifications

### Describe the problem

The bottom part of the user's notification settings says that you get these notifications for your translated languages. However, a project manager would like to receive at least suggestion, comment and pending suggestion notifications for the source strings as well. I have currently solved this by adding the source language as a language I can translate to, but this is not quite right, and this language is also displayed in my user profile.

### Describe the solution you would like

In the notification settings for managed projects (bottom part), add a checkbox that says "Also receive notifications for the source language" or something like that.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_