Handle Cantonese and Wu Chinese as separate languages

[HadrienGardeur]

Apple devices provide good support for Mandarin, Cantonese and Wu Chinese with a broad range of voices available.

That said, the language tag reported for these voices varies based on the browser:

• in Chrome:
  • Mandarin voices use zh instead of cmn
  • Cantonese voices use yue
  • Wu voices use wuu
• in Firefox all voices use zh
• in Safari, only default voices show up but Mandarin voices use zh while Cantonese voices use yue

When listing languages in Firefox, this means that all of them are grouped under "Chinese" because they all match zh.

As a bonus, we also need to make sure that voices do not show up twice if we've documented an altLanguage.
Currently, Sinji shows up in both Cantonese (language set to yue-HK) and Chinese (altLanguage set to zh-HK).