How to define a default font per language?

I wouldn't say that it's as easy as is it in Word, but it's fairly easy. Word doesn't force you to use styles, but the easiest way to do this in ID requires fairly diligent use of paragraph styles. 

My default paragraph style for mostly Arabic text with occasional English words in it would have a font specified with Arabic support (e.g. Adobe Arabic, which I actually don't like or use but we both have access to it, right? 🙂 😞

But then I could make a character style for only Latin script letters (that probably has a Latin-script font and English langauge (applied), and apply it with a GREP style:

So, you can specify exactly which characters you want it to apply to with the regular expression in the "To Text:" field. The expression there only affects A through Z, but you might want one that also handles numerals, or quotation marks, or whatever.

There are plenty of ways to specify exactly which characters get your character style applied, including specifying ranges of Unicode values. If you don't know regular expressions, you can just come back to this thread with a request and I, or one of the many other forums regulars who use regex, can figure out how to write a query for you that captures all of your Latin-script without being too greedy.

You could do it in the other direction as well - a default English paragraph style that applied Arabic font, right-to-left behavior, and Arabic language to all text that was set in Arabic glyphs. If that is what you want, let me know and I'll post some more examples.