feat(i18n): add Turkish translation (#15000)

This pull request introduces support for the Turkish language to the
frontend internationalization system and adds a new pull request
template to standardize PR descriptions. The main changes are grouped
below:

**Internationalization: Turkish Language Support**
* Added `'tr'` (Turkish) to the `Language` type and
`SUPPORTED_LANGUAGES` object in `index.ts`, including its display name,
native name, flag emoji, and resource loader.
[[1]](diffhunk://#diff-ba5f665c3490d0f5acb2cb70f08314c5373137fa8085ab05175047f10cb7fdf8L26-R27)
[[2]](diffhunk://#diff-ba5f665c3490d0f5acb2cb70f08314c5373137fa8085ab05175047f10cb7fdf8R183-R188)
* Updated `i18n-completenesses.json` to include Turkish (`"tr": 6`).

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## Summary by CodeRabbit

* **New Features**
  * Turkish language can now be selected in the app.

* **Localization**
* Initial Turkish translations added and translation completeness set to
100%.
* Locale metadata added (display name, original name, flag) for Turkish.

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