fix(native): reread the patch notes when the interface language changes

[omartelo]

The native window reads the release notes once and keeps them. Switching the interface language redrew the group names in the new language but left the items in the one they were read in, so What's new showed Portuguese headings over English notes.

keepLanguage now rereads the notes for the new language. The backend already served the right translation; only the native cache was stale.

Test

• TestChoosingALanguageRereadsThePatchNotes failed before the fix and passes now (-count=20). The fake backend in sidebar_prefs_test.go answers patchnotes.Current with the locale it was asked for.
• native/: go test ./..., go vet ./..., gofmt -l . clean.

The web window already rereads on a locale change (UpdatesSettings depends on locale).