feat: pull only languages with 50% translation completion

[OmarIthawi]

Or other percentages.

This is a core feature in Transifex. We probably need to provide a similar feature in atlas.

This is closely related to:

• feat: Add default languages list in atlas (in discussion) #30

• List of supported languages by Open edX.

[OmarIthawi]

In Sept 20th, 2023 meeting we've decided to set the threshold to 50%.

So all languages which have 50% or more completion rate should be included by default in atlas pull when --filter isn't used.

[OmarIthawi]

Closing this task for now. The current status is that we pull all the languages and atlas so far is pretty fast with that.