changing misleading language identifier

fixes issue #4251
Typical language standards use dashes, but symfony uses underscores like defined in ICU. So the docs were misleading.
Thanks for the link. In my opinion is a bit "rough". I was thinking about something like:

| Code       |  Locale
| ------------ | -----------------------------
| `es_MX`   |  Spanish (Mexico)
| `fr`            |  International French
| `...`           |
| `zh_Hant` |  Chinese (Traditional Han)