Allow DeepLTranslator supports `auto`

[jasl]

trafficstars

According to https://developers.deepl.com/docs/api-reference/translate/openapi-spec-for-text-translation

source_lang is optional, and DeepL will auto-detect language if it's not given.

Note: I don't have Unity or C# development environment, so I can't compile it, but the code is simple