Extension Developers can provide their extension in whatever langauge they want.

But Send Extension chat doesn't support translations.

Please extend https://dev.twitch.tv/docs/api/reference#send-extension-chat-message to support translations

So instead of

{
  "text": "Hello",
  "extension_id": "uo6dggojyb8d6soh92zknwmi5ej1q2",
  "extension_version": "0.0.9"
}

as the payload

support

{
  "texts": {
"en": "words and stuff",
"de": "the german transation",
"fr": "the french translation"
   },
  "extension_id": "uo6dggojyb8d6soh92zknwmi5ej1q2",
  "extension_version": "0.0.9"
}

And then the Twitch frontend will select whatever language the user is using and default to broadcaster language if a matching language not found.

or support a "default"

{
  "texts": {
"default": "words and stuff",
"en": "words and stuff",
"de": "the german transation",
"fr": "the french translation"
   },
  "extension_id": "uo6dggojyb8d6soh92zknwmi5ej1q2",
  "extension_version": "0.0.9"
}

Naturally use/support the language codes that language uses in the query string parameters from https://dev.twitch.tv/docs/extensions/reference#client-query-parameters