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Translator Engine Overview

What exactly is the difference between Google, Microsoft and DeepL?

Google

The most famous translation engine, it has the most available languages and most people consider it to be more accurate than Microsoft, although there is more nuance to this, as in my experience Microsoft often ends up being more accurate (Russian for example)
Google Translate is the fastest translator available.

Microsoft

Quite a bit less well-known, and has a reputation for not being great. As mentioned prior, I think Microsoft is often more accurate than Google, although for certain languages I imagine Google is more accurate, if you have access to both it's probably worth trying each one out for a few days and seeing which works better for your language needs.
Microsoft Translate is also slower than Google.

DeepL

The newest addition to the block. DeepL has a very limited selection of languages, basically just the main ones, however, its translation quality is far higher than either of the other available services. If you have access to it, and the languages you are translating to and from are part of its repertoire. I'd recommend using it.
It is however even slower than Microsoft Translate.

Conclusion

If you need accuracy, go for DeepL. If you need speed, and lots of languages go for Google. If you need a mix of both, go for Microsoft.