I think that the first one for "We directly translate Chinese to English." sounds better. I think it may be because we native speakers often don't entirely pay attention to adjectives. So, putting it before the noun makes us pay more attention to it. :)
And for the last example I'd go for "He can only play the piano." for the same reason. It just makes me pay more attention to that one little detail more. But, since that detail is important and not having it there changes the sentence completely, its better that I actually pay more attention to its meaning.
- For Teachers