I came here to say the same thing. I live in Malaysia and while I know a lot of Malay I often want to translate some text. It's annoying that the auto-detect rarely detects the Malay so I have to actually tell it "THIS IS MALAY! LIKE LAST TIME! LIKE EVERY OTHER ^$%# TIME! IT'S %^$# MALAY!"
And then it translates it just fine.
Having a favorite or default language seems such a no-brainer that it's actually annoying that there isn't one. Instead of being happy to have a built-in translator that avoids me having to find Google Translate, I find myself swearing at the thing.
This is not a good user experience.
I can understand it not being able to auto-detect the slang and abbreviations Malaysians use - Google Translate can only figure out most of the words too -but that's all the more reason to offer a default.
Realistically nobody is out there just casually browsing the world's websites in languages they don't know and have no relationship with. In the real world we only really want to translate the language of the new country we're living in, visiting or interested in, so the vast majority of users would benefit from being able to select a default language, at least for the duration of their holiday or whatever.