What does it mean "declare English target fonts before Chinese target fonts, because English language fonts do not contain the glyphs for Chinese characters, but Chinese fonts do contain a-z characters..."
A user-agent (mail client, web browser, etc.) chooses a max of 2 fonts that may be the source of a given glyph:
- the first font in the font-family list that is available as either a webfont, locally installed custom font, or system font (pre-installed in the OS)
- a fallback font, typically one of the OS-native fonts; it’s possible for there to be no fallback font, though
So let’s say you want to display the CJK ideograph 水 (Unicode character 6C34) and your text also has the simple Latin-1 character W.
Since you don’t want to rely on the fallback font having either of those characters, you want the user-agent to choose a font that has both of them. A CJK-focused font like MS Mincho has both; a generic font like Arial only has the W (though modern Arial does have the glyphs for many other languages, like Hebrew and Arabic, just not CJK).
So if you put Arial first, as Arial is present on almost every machine, the system will stop there. But then it won’t have a glyph for 水. If MS Mincho is first, it’ll be able to supply both glyphs from that font.
Note: the idea that “Chinese fonts do contain a-z characters, but not vice versa” (which would better phrased as “Chinese fonts do contain Latin-1 characters...”) is indeed true in practice, but it’s not defined by a standard or anything. It‘s quite possible for a font to only have Chinese, Japanese, and Korean characters and punctuation; it’s just rare.
You also aren’t obliged to use the same font for all visitors. I definitely wouldn’t try to use MS Mincho everywhere. You want to choose a font that fits the distinct cultural styles of each of your audiences, meaning your font-family cascade should be different depending on the page’s locale/language.