> Is that [ease of typing it on any keyboard] an inherent property of English, or because we haven't invested enough effort into making non-English keyboards better?
Not of English inherently, more a feature of a language that uses a small set of symbols with many different combinations. The simplest and easiest approach is to give each button a symbol; it's also typically faster than typing on, say, some sort of touch-input keyboard. I agree that there is demand, but English still makes sense for development.
> People do write Romanized Hindi... in India, but that has more to do with the lack of native keyboards than anything else.
And I am stuck using SI units for science, even though I have a preference for American units. Compatibility is important, and as you just stated, people have adapted.
> Where is this unwillingness to support multiple languages in software coming from, anyway?
Immense difficulty for minimal return. About two months ago, and article was posted about just how difficult it can be: https://gankra.github.io/blah/text-hates-you/
Even if many programmers aren't dealing directly with the rendering, UI design is a huge pain. Coupled with the fact that many countries with languages more difficult to support have fewer users, it can be a simple business decision.
I can see why a consumer-friendly interface probably should have multi-lingual support (especially if it has a large, non-English user-base), but development tooling operates at a lower level where more computer-friendly representations are necessary. It also has to be able to all work together under the hood, which requires a common language. English ended up the lingua franca, partly because computers were developed largely in America and partly because English text happens to simply be easier to represent than other languages, at least for computers.