Add dedicated `lang` parameter

We have a project with complex json structures as a result from many nested relationships. We relied on the &lang= query param from v8 to acquire the corresponding translations, but we now see that v9 does NOT have anything similar, as far as we've seen. The only similar feature to that would be the &deep[translations][_filter][language]=es-es query param , but this makes us define the filter for every nested items, which is not what we expected when thinking about migrating to v9. Is there any feature similar to the &lang query param in v9 that we're not aware of?
If not, it'd be a nice idea to introduce it, because I assume that a lot of people will be in the same situation as us.

What we want to do
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What we have to do
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