How do you create routes with optional parameters in Sapper?

Let's say I have a /foo route. But sometimes people hit /foo with a language parameter: /fr/foo. And other times they might hit it with a language and a country: /ca/fr/foo

So I need a routing table like

[country]/[language]/foo 
[language]/foo 
/foo 

That all direct to the same page.

Should I create a tree like this?

src/routes
└── [country]
    └── [language]
        └── foo.svelte

If that's the answer then how do I direct [language]/foo to [country]/[language]/foo?

I don't see any optional params in the docs

Optional parameters aren't yet supported, but you can almost fake it with 'rest routes' (which I've just realised aren't yet documented 🤦‍♂️) — routes/[...parts]/foo.svelte would render foo.svelte with a page.params.parts array that contained the preceding segments.

Unfortunately that doesn't match /foo, which it arguably should. I've raised an issue: https://github.com/sveltejs/sapper/issues/765