docs: document phonemis submodule init for demo apps

[msluszniak]

Description

The TTS pipeline depends on the phonemis git submodule under packages/react-native-executorch/third-party/common, which isn't pulled by a plain git clone. The demo-app instructions in the root README and all translated READMEs (readmes/README_{cn,es,fr,in,pt}.md) skipped this step, so running any TTS-touching demo fails out of the box. This PR adds the scoped git submodule update --init packages/react-native-executorch/third-party/common step to each README. Scoped to .../common instead of --recursive so we don't pull tokenizers-cpp and googletest, which the demo apps don't need.

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

1. Fresh-clone the repo without --recurse-submodules.
2. Follow the updated Demo Apps section.
3. Confirm the speech app builds and runs end-to-end.

Screenshots

Related issues

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes