[Screenshots] Mocked notifications are always in English

Parent: #17128

When running the WPScreenshotTest and JPScreenshotTest to capture Play Store screenshots, we mock the network requests, including the ones returning notifications to be shown in the Notifications tab.

Those mocks always return English copies though, which means that this screen shows notifications in English ("X and 5 others liked your post")… even when capturing screenshots for non-en-US locale.

This makes those screenshots inconsistent with the reality, and could make users think, looking at the Play Store screenshots, that the Notifications screen is not localized (while in the real, non-mocked world, they are).

Warning: Those tests were run from the branch from this PR (which fixes screenshot automation). If you start working on this, make sure that either that PR has landed since, or that you cut your branch to fix this from this PR's branch, to benefit from the tooling fixes related to screenshots that I've made so far.