Brave Today: support Japanese content feed

[petemill]

Description

When the user's OS is set to Japanese language, we should load the Japanese content feed. This feed contains a different set of sources, mainly local to Japan.

Steps to Reproduce

Localised content is shown

1. Set OS to Japanese

2. Restart Brave

3. Open NTP and scroll down.
   Expected: Brave Today has all japanese content

4. Set OS to English or French or any language except Japanese

5. Restart Brave

6. Open NTP and scroll down.
   Expected: Brave Today has all English / default sources content

Brave Today is enabled by default for Japanese language users, as is currently so for English language users only

1. Quit Brave
2. Set OS to Japanese
3. Start Brave with clean profile
4. Open NTP
   Expected: Brave Today should be showing / enabled by default

Notes

When switching between English and Japanese OS languages, upon restart we expect the browser to load the expected feed sources (default "english" content sources for all languages, and "japanese" content sources only for japanese language. However, when switching to a localized feed which the user has made changes to previously (i.e. turning publishers on or off), those same changes should still be respected.

[stephendonner]

Verification PASSED on macOS beta using build

Brave	1.22.55 Chromium: 89.0.4389.72 (Official Build) beta (x86_64)
Revision	3f345f156bfd157bd1bea06310e55f3fb2490359-refs/branch-heads/4389@{#1393}
OS	macOS Version 11.2.3 (Build 20D91)

Prerequisites (macOS):

• Set OS language to Japan
• Set OS region to Japan
• Set OS date/time format to Japan

Test Case #1 (clean profile)

• ensured that the Japan source is being used for Brave Today
• ensured that Japanese sources are being displayed under the Customize Dashboard via Brave Today
• ensured that disabling Brave Today via Customize Dashboard works as expected
• ensured that you can restore Brave Today to the default state via Reset Brave Today sources

Example	Example	Example

Test Case #2 (switching between Japan/United States)

• ensured that the Japan source is being used for Brave Today when Japan is being selected as the default locale/language
• ensured that the USA source is being used for Brave Today when United States/English is selected as the default locale/language
• ensured that the source selection settings are remembered when switching between the two locales
  • Disabled all feeds for Technology & Crypto using Japan
  • Enabled all feeds for Technology & Crypto using United States
  • ensured that switching between the two sources saves the above selections

Additionally, I filed or asked about issues which were filed as:

• Brave Today: support Japanese content feed #14306
• https://github.com/brave-intl/brave-today-aggregator/pull/116
• Brave News error message needs translated strings #14510