I was testing EPiServer.Labs.LanguageManager for a client's site, and got an error as soon as I tried to translate an XhtmlString property.
The Error log says "The maximum request size has been exceeded", "Cannot translate with Cognitive service".
I found this regarding limits for translator: https://docs.microsoft.com/en-us/azure/cognitive-services/translator/request-limits
It seems that there is a limit on 5000 characters per request, which you quite easily exceed, especially if you have created a table in the rich text editor.

It shouldn't be a super big task to break up the characters in multiple request, but might need some thinking regarding how to break up the html, so that the translator knows what is markup and what is actual text to translate.