Nested JSON/YAML/PHP language files fail to load; fixtures suggest nesting is supported #1371

The test/fixtures folder contains nested fixtures for three formats, but Language::load() only accepts a flat key => string map. Nested files fail:

$lang = new Language('en-GB', 'en-GB');
$lang->load('test/fixtures/json/en-GB.json', 'json', 'en-GB');
// TypeError: Language::addString(): Argument #2 ($string) must be of type string, array given

The same error occurs for php/en-GB.php (WINDWALKER => [TEST => [SAKURA => ...]]). YAML (yaml/en-GB.yml) also fails for a different reason: Class "Symfony\Component\Yaml\Yaml" not found. symfony/yaml is not required by windwalker/language, and it is not installed in a default setup.
Only the INI fixtures are used by tests (test/LanguageTest.php:32-33, 324). The JSON, PHP, and YAML fixtures are not referenced by any test.
Questions to resolve:
1.
Is nested key flattening intended? If so, Language::load() or addStrings() must flatten arrays into dot-separated keys, and this is currently missing. If not, the nested fixtures are stale and should be removed or converted to flat form.
2.
If YAML is supported, symfony/yaml should be declared in suggest (or require) of packages/language/composer.json, and the YAML fixture should be tested only when present.
Expected behavior for the decision: nested windwalker.language-test.sakura in JSON should resolve to windwalker.language.test.sakura, matching the INI fixture key.