Google Translate uses English as an intermediary?

Bilingual dictionaries often don't exist for pairs like Greek and Thai, so in order for translation software to work, a third language would be needed. This is similar to the problem of UN interpreters who only interpret to and from the UN's six official languages. Other interpreters are needed who speak a UN language and other languages. 

How the UN Translates Everything in Real-Time

To translate from Greek to Thai, you need either: someone who speaks both languages or a Greek speaker and a Thai speaker who are also bilingual in some other language. This is how we ended up with lingua francas like Latin in the past or English now. 

I suspect that translations to and from English are probably more accurate than translations that require English as a middle step. I doubt very much Google has or has made bilingual dictionaries of every language pair.