… (javiereguiluz)

This PR was squashed before being merged into the 4.2 branch (closes #31786).

Discussion
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[Translation] Fixed case sensitivity of lint:xliff command

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | -
| License       | MIT
| Doc PR        | (not needed)

I was checking some errors of `lint:xliff` (https://travis-ci.org/EasyCorp/EasyAdminBundle/jobs/540053551#L657) and saw this:

```
ERROR  in src/Resources/translations/EasyAdminBundle.sr_RS.xlf
 * There is a mismatch between the language included in the file name ("EasyAdminBundle.sr_RS.xlf") and the "sr-rs" value used in the "target-language" attribute of the file.

ERROR  in src/Resources/translations/EasyAdminBundle.zh_CN.xlf
 * There is a mismatch between the language included in the file name ("EasyAdminBundle.zh_CN.xlf") and the "zh-cn" value used in the "target-language" attribute of the file.
```

This was suspicious, so I checked the XLIFF standard and it says (http://docs.oasis-open.org/xliff/v1.2/os/xliff-core.html#target-language):

```
target-language:
  Unlike the other XLIFF attributes, the values are not case-sensitive.
```

So, it's valid that `zh-cn` is the target language and `zh-CN` is the file extension. This PR fixes that.

Commits
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ec690b2 [Translation] Fixed case sensitivity of lint:xliff command