custom background for NTP customization strings are not translated

[btlechowski]

Follow up to #19640

Steps to Reproduce

1. Clean profile
2. Open Brave in Polish (PL) locale
3. Open NTP
4. Click Dostosowywanie

Actual result:

Upload from device is not translated
Use your own is not translated

Expected result:

Upload from device is translated
Use your own is translated

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Brave	1.37.83 Chromium: 99.0.4844.51 (Official Build) beta (64-bit)
Revision	d537ec02474b5afe23684e7963d538896c63ac77-refs/branch-heads/4844@{#875}
OS	Ubuntu 18.04 LTS

cc @rebron @simonhong @brave/legacy_qa @mkarolin

[kjozwiak]

Removing from 1.37.x as it's not labelled as release/blocking. If this ends up being fixed with the l10n order, we'll need to add/mention this issue within the uplift PR. If this needs to be fixed in a separate PR (not the l10n order), it should go through the normal process of landing in master and then creating an uplift at which point, this issue will be moved into 1.37.x.

[btlechowski]

Verification passed on

Brave	1.37.109 Chromium: 100.0.4896.60 (Official Build) beta (64-bit)
Revision	6a5d10861ce8de5fce22564658033b43cb7de047-refs/branch-heads/4896@{#875}
OS	Ubuntu 18.04 LTS

Verified test plan from the description