I'd tell the client what both versions cost and let them decide. If they pick the non-pro version and regret it later, it was their decision.

I suppose it depends on how much is being charged for "all those obscure accents".

Obviously, if the type designer is constrained to expend effort on covering a wide range of languages using the Latin script with his font, that labor needs to be paid for. If this is done by making an extensive range of accents the dividing line between the regular and "Pro" versions of a font - the way that a Xeon or Opteron processor has features that a consumer processor does not, and comes at a considerable price premium - then those who don't need them won't buy them.

they’re sure they will only deal with English-speaking regions

In this day and age, that’s presumptuous.
What about, just to make an example, clients’ names?
That said, I found that the first obstacle to a more-than-just-ASCII typography (which includes, other than diacritics, proper quotations, etc.) aren’t fonts, but software support and input methods.

It could be an overtly naïve attitude vis-à-vis accepted études in loanwords.

Well, common loanwords usually just use the basic European accents found in ISO 8869-1, which usually don't cost extra.

Theunis de Jong and riccard0, yes, a writer in today's world will need that stuff.

A straightforward solution would be, of course, a modular/incremental upgrade pricing, along the lines of webfonts’ subsetting.

The client is always right.

modular/incremental upgrade pricing

I haven’t seen that coming very often. On the contrary, I’ve seen several foundries’ licensing terms that explicitly denies possibility for upgrading. Basically you just don’t pay extra to upgrade; you pay the full amount again.

??
This almost sounds like pricing fonts by the glyph. Does anyone do that?

For what it is worth, language support is the same in all my fonts, "pro", free or otherwise. It's the OT features and general complexity that increase prices. Why would I charge someone who needs some of those “obscure foreign accents” more than I’d charge anyone else? Or put another way, how can I expect to sell any fonts in Turkey or Esperantia if I ignore language support?

From the client's perspective, it also depends on the application. In two recent projects, I commissioned signage versions of two typefaces for my employer. I didn't specifically say to the designers, "Don't include all those obscure foreign accents." but I did suggest that for at least one of the typefaces that the designer could limit language support to Latin 1 - But not because I thought I'd save anyone any money. Since our signs will only ever be set in English and French, by the time the projects passed all the hurdles of getting approved, the time to actually do the work was getting really tight and it seemed like a logical place to save a little bit of time – Although how much time, is a good question. They'e components. They update automatically.