Fix localization files generation

[ssenchenko]

Description of what this PR is changing or adding, and why: fixing incorrect or outdated documentation. Localization files are not generated automatically on flutter run, they must be generated by running flutter gen-l10n. At least for 3.x.x version.

Issues fixed by this PR (if any): 7434

Presubmit checklist

• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [x] This PR uses semantic line breaks of 80 characters or fewer.

[google-cla[bot]]

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[domesticmouse]

Please sign the CLA. We can't progress this PR until it is signed.

[ssenchenko]

Hi there! Can I get a review please? All checks are okay