Regression in a38ae914.

The Subquery class which Exists inherits from defaulted to using single-valued
querie's field if no output_field was explicitly specified on initialization
which was bypassing the Exists.output_field defined at the class level.

Moving Subquery's dynamic output_field resolution to _resolve_output_field
should make sure the fallback logic is only performed if required.

Regression in 08654a99.

Thanks Oli Warner for the detailed report.

Thanks James Davis for suggesting the fix.

Thanks Florian Apolloner for assisting with the patch.

Follow up to 841a8778.

Fixed #29144 -- Made untranslated strings for territorial language variants use translations from the generic language variant.

When __len__() is called (e.g. when casting to list or tuple), the
QuerySet is evaluated and the result cache populated. iterator()
shouldn't be called on the QuerySet after that, as it would reset the
result cache and trigger a second query.

Removing all() in __iter__() prevents a duplicate query when choices are
cast to a list and there's a prefetch_related().

Regression in 5cc28dc7.

Forward port of 2126e931 from stable/2.0.x.

Thanks Matthew Pava for the report and Tim Graham and Carlton Gibson for
reviews.
Regression in 19b2dfd1.