I had a hard time understand what this is supposed to do, so for those that also did not understand, I think it -
Tells the browser that the page the href attribute links to should be translated to a specific language by the browser translation feature.
So this is basically to maintain language consistency when the previous page knows for sure that it links to a page in a different language then the language the user expects/understands.
I hope it will come with an opt-out and/or smartiness like, if they user chose never to translate language X, it will be respected regardless of hreftranslate="Y", or if the user does not want to read machine-translated content at all (because it is not awesome for their language or for any other reason), no translation will be automatically employed.
Also, try not to advocate camelCased HTML attributes, this is not React or an old MSDN example. ;) :P