fixed locale management in sub-requests

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6932, #5532, #8604
| License       | MIT
| Doc PR        | n/a

Same fix as #8821 but for `_locale`, and a rebase of #8604.