I'm an iOS/Android app developer, and inspired in part by Apple's recent iBooks font overhaul, I've started looking at how I might improve the typography in my own app. I've been reading up a lot on typography the last few weeks, but I'm still a total novice at this stuff and I'm hoping people here might be able to point me in the direction of some specific fonts (or at least some approaches to font selection) that might work well for it. It's actually a pretty successful app and I'm OK with spending some money to license a really great font, but a free font recommendation would be lovely too assuming that the license allows for it to be bundled with a non-open-source app.
My app is a Chinese-English translation dictionary, and while originally I was mostly looking at improving the font we use for Chinese (replacing the decidedly ugly built-in [[http://en.wikipedia.org/wiki/East_Asian_sans-serif_typeface|Hei-style]] ones with a nice and readable [[http://en.wikipedia.org/wiki/Ming_(typefaces)|Ming-style]]), I've realized that I really ought to consider replacing the font used for English text too; I'd like something that's distinctive, looks good next to a Ming-style Chinese font (so probably something serifed) and that's well-suited for the type of reading one does in a dictionary.
Also, Chinese text in dictionaries is usually accompanied by pronunciation guides, written in a system called Pinyin - uses the Roman alphabet along with diacritical marks to indicate which of the four tones a particular syllable is in (macron / acute / caron / grave). So I'd like to either find a third font that I can use for that (something that goes well with the other two, but is distinctive enough to make it easy to distinguish from the other non-Chinese font) or just choose an English font that's also diacritic-friendly and remains very readable at different weights (so that I could, say, make the Pinyin semibold and the English regular).
So: does anyone have any recommendations for fonts I should consider? I'm in this for the long haul, so I'm primarily focusing on Retina / XHDPI displays and am OK if a font looks a bit fuzzy on lower-resolution smartphones.
I'm also looking for more general advice on how to style / space text in a dictionary entry; they generally have a lot of different sections that you're trying to separate - parts of speech, fields of use (biology, law, etc), styles (old-fashioned, obscene, slang), usage notes, example sentences (which in a Chinese dictionary combine Chinese characters, Pinyin, and English), etc - but using a different font (or even just a different style) for every one of these seems liable to give my users seizures. How do I separate a lot of different types of information in a single page of fairly-dense text on a smartphone screen?