I'm developing a Smart Home skill with locale set to en-US, although all devices in my test device cloud are physically in Europe. Nowhere in the docs is this deemed an impossibility.
Based on my skill being en-US, I assume that all of the capability interfaces listed under 'supported locales' as supporting en-US at https://developer.amazon.com/docs/device-apis/list-of-interfaces.html should work for me, right?
Well, some do and some don't.
Contact sensor and motion sensor work fully as expected for my en-US skill and all endpoints with these capability interfaces are discoverable and reportable.
However, other en-US ONLY capabilities such as toggle, mode, range and doorbelleventsource cause discovery to selectively and silently fail with my device cloud. Probably because my echos are not linked to a US amazon account but the docs don't say that on the page in question. Amazon only mentions locale as needing to be en-US. It is in my case. So it *should* work.
Amazon needs to address locale vs physical location specifically in the docs. I'm also very curious why some interfaces work and others don't. Also, silent failure is not developer-friendly. A json response detailing what endpoints are being rejected and for what reason would be a real help in debugging. RIght now, the only way to detect something is not being accepted is to look at the post-discovery json to see what's missing. No error or warning message is given.
On a humorous note: I'm writing this on Sunday 18th May. Let's see if I'm right and the first response from Amazon is in 8 days time on 26th :-)