Switch use of translate::kUnknownLanguageCode to language_detection::

This is a no-op refactor.

This is split from https://crrev.com/c/6048141 and covers

components/contextual_search

This CL was uploaded by git cl split.

Bug: 380786760


Switch use of translate::kUnknownLanguageCode to language_detection::

This is a no-op refactor.

This is split from https://crrev.com/c/6048141 and covers

chrome/browser/ui/translate

This CL was uploaded by git cl split.

R=gangwu@chromium.org

Bug: 380786760
Change-Id: Ibeb9488b8dc61f24959ec494dae95aab36a0d75f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6161322
Reviewed-by: Gang Wu <gangwu@chromium.org>
Reviewed-by: Trevor Perrier <perrier@chromium.org>
Auto-Submit: Fergal Daly <fergal@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1404641}