docs: Add language-agnostic for Playwright testing doc

[hienha-sw]

This pull request adds a comprehensive documentation guide for language-agnostic testing in Playwright using the @shopware-ag/acceptance-test-suite. The guide explains how to write tests that adapt to different locales by using translation keys and provides step-by-step instructions for integrating and customizing translation support in your own projects. It covers usage patterns, environment configuration, translation file organization, and common troubleshooting tips.

Language-Agnostic Testing Concepts:

• Introduces the translate() function and the Translate fixture for replacing hardcoded strings with translation keys in tests and page objects, enabling tests to work across multiple languages.
• Explains translation key patterns, locale file structure, and provides example translation files for English and German.

Custom Integration and Extension:

• Details how to extend the base test suite with custom translation fixtures, merge translation resources, and define type-safe translation keys for IntelliSense and compile-time validation.
• Describes how to organize locale files, create an index for imports, and merge custom translations with the base suite.

Environment and Configuration:

• Provides instructions for configuring Playwright to support dynamic language switching via environment variables and browser arguments, ensuring the browser UI matches the test language.
• Lists common issues and troubleshooting steps

[Copilot]

Pull Request Overview

This PR adds comprehensive documentation for language-agnostic testing in Playwright using the @shopware-ag/acceptance-test-suite. The guide enables developers to write tests that work across multiple languages by using translation keys instead of hardcoded strings.

• Introduces the translate() function and Translate fixture for creating locale-aware tests
• Provides detailed integration guide for custom projects with translation support
• Documents translation key patterns, file organization, and environment configuration

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[Copilot]

Environment variable assignment should use uppercase 'LANG' for consistency with other examples in the document and standard Unix conventions.

Suggested change

	lang=de npx playwright test
	LANG=de npx playwright test

[Copilot]

The environment variable check order should prioritize 'LANG' over 'lang' since 'LANG' is the standard Unix environment variable, and lowercase 'lang' appears to be a custom variable that's inconsistently used.

Suggested change

	let lang = process.env.lang || process.env.LANGUAGE || process.env.LANG || 'en';
	let lang = process.env.LANG || process.env.LANGUAGE || process.env.lang || 'en';