I want my software available to a worldwide audience, and so am a bit concerned about fonts used in my program's GUI (Winforms, .NET3.5, C#).
Let's assume I choose "Microsoft Sans Serif" for everything (size 9 I've heard https://stackoverflow.com/a/7045886/848344 is better than the default 8.25 size for some reason). Now, I understand that 'fallback' fonts are used automatically when the character isn't available in Microsoft Sans Serif. So a block of text that's supposed to be just "Microsoft Sans Serif" may simultaneously also use "Arial Unicode MS" and "SimSun-ExtB". These three fonts seem to cover most of the bases.
Unfortunately, some systems may lack one or more of these fonts (or not have the latest version). For example, Arial Unicode MS is only supplied with Office. Also, WinXP (no SP) only supports Microsoft Sans Serif - v1.33, whilst XP SP2 uses v1.41, and Vista/Server 2008 has v5.00. Later versions of Microsoft Sans Serif offer improved Cyrillic, Latin and Arabic support.
However, do I need to worry about this? Can I assume that either these type of users will have upgraded to at least XP SP2 or failing that, have a font which Winforms can fall back to which supports any missing characters? Surely if my program goes wrong for them, then ANY program would also be wrong for them, so maybe they have the appropriate fonts (including ones I haven't listed) that Winforms will fall back to anyway?
In summary, can I just use Microsoft Sans Serif (size 9) in a Winforms app, and not worry about worldwide font support (due to the automatic font fallback) if I want to cover at least 90-95% of users (home and business)? Bear in mind, things don't have to be perfect, as my app is more of a read/display type of app rather than an editing one, though maybe any answer could cover both cases for other people who need extra accuracy.
What you described is made a non-issue by the font fallback mechanism which is built into the platform and by the fact that, should a user wish to use a given language, such a user would have installed the necessary language support components including fonts and IMEs.
There are plenty of other things to be aware of with regard to making a world-ready application, though. One book that covers all this (but is quite old now) is Microsoft's Developing International Software. One of these, which you alluded to, is the need to use bigger fonts so that glyphs such as Chinese are legible (doesn't mean you need bigger fonts in all languages, but you need to make it possible to increase font sizes for languages that need it). Another big one is the issue of locale-specific formatting for dates and numbers. A fun one is that capitalisation is locale-specific, as illustrated by the Turkish i problem.