Add a checkbox to exclude a language from stats

[jdimeo]

Is your feature request related to a problem? Please describe.
We use custom language codes for "back translation". https://www.smartling.com/blog/what-is-back-translation-and-why-is-it-important
However, these language codes are considered "real" languages and so our projects always look to be never complete:

The majority of that 74% are these blank language codes. Our users mostly care about the normal "forward" translation progress.

Describe the solution you'd like
Add a checkbox on the language settings page that says something like "Exclude from project statistics" so that the state of those languages are not included in % translated, % untranslated, etc.

Describe alternatives you've considered
A fancier solution would be some kind of custom "language type" to group languages by forward translation vs. back translation and possibly show separate statistics for each group, but I don't think that would be a generally useful feature to other users.

Additional context
Additionally, it would be great if other emoji that weren't flag emoji were still displayed instead of just shown as "X". Not a big deal but helpful for custom languages. I tried using 🔙 in this example above.

NOTE: I wouldn't expect this to affect Tasks. If we order a Task with these custom language codes, they should use stats to report progress on the task, etc. - this is just for the "default" project stats.