We have used Language Intelligence, we were very impressed by their project management and quality at the end of the project.  https://www.languageintelligence.com/

We are building a Google Extension for Google Sheet using APIs so one can translate using Blend https://www.getblend.com/

This is not a third party vendor as such, but if you wanted to handle automatic translations in-house, I would highly recommend DeepL. We’ve been using their machine translation for a couple of years now and the quality is excellent, exceeding any of the other machine translators we’ve tried such as Google Translate.

https://www.deepl.com/en/translator

I’ve had good very experiences with Lionbridge in the past. They outsource translations to regional/native speakers (ideal for complex and technical content), and on the whole have been great to work with. https://www.lionbridge.com/