i18n-ally:

Inline Annotations

source: https://github.com/lokalise/i18n-ally/raw/screenshots/annotation-animated.gif?raw=true

Hover and Direct Actions

source: https://github.com/lokalise/i18n-ally/raw/screenshots/hover.png?raw=true

Manage All Translations in One Place

Source: https://github.com/lokalise/i18n-ally/raw/screenshots/review-sidebar.png?raw=true

Editor UI & Review System

source: https://github.com/lokalise/i18n-ally/raw/screenshots/review-editor.png?raw=true

Extract Translations from Code

source: https://github.com/lokalise/i18n-ally/raw/screenshots/extract.png?raw=true

Report Missing Translations

source: https://github.com/lokalise/i18n-ally/raw/screenshots/problems.png?raw=true

Machine Translation

source: https://github.com/lokalise/i18n-ally/raw/screenshots/quick-actions.png?raw=true

Annotations for JSON and YAML

source: https://github.com/lokalise/i18n-ally/raw/screenshots/annotation-locale.png?raw=true

General Features

• Supports multi-root workspaces
• Supports remote development
• Supports numerous popular frameworks
• Supports linked locale messages
• Uses i18n for the extension itself, of course. Translation List

Multilingual Support

This extension itself supports i18n as well. It will be auto-matched to the display language you use in your VS Code editor. We currently support the following languages.

• English
• Simplified Chinese (简体中文)
• Traditional Chinese (繁體中文)
• Norwegian (Norsk)
• Spanish (Español)
• Japanese (日本語)
• Dutch (Nederlands)
• Portuguese (Brazilian)
• French (Français)
• Swedish (Svenska)
• Ukrainian (Українська)
• Russian (Русский)
• German (Deutsch)
• Thai (ภาษาไทย)
• Turkish (Türkçe)
• Korean (한국어)
• Hungarian (Magyar)

Supported Frameworks:

Supported frameworks are auto-detected when a matching dependency is found in the project.

source: https://github.com/lokalise/i18n-ally/wiki/Supported-Frameworks

I am not yet using it in my Flutter project, but I'm planning to integrate it soon.