Translations are missing for locales that include region (only on Android)

[fluiddot]

Description

On Android, the strings shared between the native and web version are not translated for the locales that include regions in their value (i.e. zh-tw).

Step-by-step reproduction instructions

1. Change the device language to Chinese/Taiwan (i.e. zh-tw).
2. Open the app.
3. Open a post/page.
4. Tap on the ➕ button located in the toolbar.
5. Observe that the block titles within the block picker are not translated.

Expected behaviour

All strings shared between the native and web version should be translated, if the translation is available in the Gutenberg GlotPress project.

Actual behaviour

The strings shared between the native and web version are not translated for locales that include the region in its value (i.e. zh-tw).

Screenshots or screen recording (optional)

Using zh-tw locale:

Post title placeholder	Block picker

WordPress information

• WordPress version: N/A
• Gutenberg version: N/A
• Are all plugins except Gutenberg deactivated? N/A
• Are you using a default theme (e.g. Twenty Twenty-One)? N/A

Device information

• Device: Samsung Galaxy S20 FE 5G
• Operating system: Android 11
• WordPress app version: trunk

[fluiddot]

The list of potential languages affected by this issue are the following:

• en-au - English (Australia)
• en-ca - English (Canada)
• en-gb - English (UK)
• en-nz - English (New Zealand)
• en-za - English (South Africa)
• es-ar - Spanish (Argentina)
• es-cl - Spanish (Chile)
• es-cr - Spanish (Costa Rica)
• nl-be - Dutch (Belgium)
• pt-br - Portuguese (Brazil)
• zh-cn - Chinese (China)
• zh-tw - Chinese (Taiwan)

UPDATE: Since we use the non-region locale as fallback when the translation file is not found, some of the previous languages were unaffected. This is not the case for zh-cn - Chinese (China) and zh-tw - Chinese (Taiwan) as there's no fallback for Chinese.

Using en - English as fallback

• en-au - English (Australia)
• en-ca - English (Canada)
• en-gb - English (UK)
• en-nz - English (New Zealand)
• en-za - English (South Africa)

Using es - Spanish as fallback:

• es-ar - Spanish (Argentina)
• es-cl - Spanish (Chile)
• es-cr - Spanish (Costa Rica)

Using nl - Dutch as fallback:

• nl-be - Dutch (Belgium)

Using pt - Portuguese as fallback:

• pt-br - Portuguese (Brazil)
  NOTE: pt has less translation coverage so this language is probably higher affected.