Lingvanex is a translation engine Brave uses for their website translation feature.
Transifex is a localization management platform mainly used for localizing user interface. It’s like Github, but for translating stuff instead of coding stuff.
They offer many features, but availability will vary depending on third-party implementation (aka how much money Brave is willing to spend on the service).
Only full page translation is currently supported. I have not seen anything about supporting partial translation (like Vivaldi) or on the fly translation (like the Google Translate and Deepl extensions).