[1.6.x] Ensured that BoundField.as_widget always returns properly localized fields.

This is a follow-up to #18777 which improperly converted to strings in
prepare_value and as such caused regressions like #21074.

Refs #18777, #21074

Backport of 56743cf9e337826e4c615909570bb057142a6a7b from master.