Chrome Version: 52.0.2743.116 (Official Build) (32-bit)
Chrome OS Version: 8350.68.0 (Official Build) stable-channel nyan_big
Chrome OS Platform: Acer Chromebook 13 C810
Network info: not relevant

Please specify Cr-* of the system to which this bug/feature applies (add the label below).
Cr-UI-Internationalization?

Steps To Reproduce:
(1) Look at the ComposeKey extension on the Chrome Web Store (https://chrome.google.com/webstore/detail/composekey/iijdllfdmhbmlmnbcohgbfagfibpbgba)
(2) See that it does not support any keyboard layout other than US English

Expected Result:
I should be able to designate a compose key on any keyboard layout (e.g. right alt on UK Querty).

Actual Result:
If I want a compose key, I am forced to use the US keyboard layout.

How frequently does this problem reproduce? (Always, sometimes, hard to reproduce?)
Always

What is the impact to the user, and is there a workaround? If so, what is it?
It is difficult to type symbols that are not on the keyboard without switching to another keyboard layout. Other issues that attempt to deal with this problem (e.g. 179473) appear to be dead - that particular issue has not been touched in over 6 months.

Using right alt/alt gr as a dead key is not a solution, as it does not support typing, for example, en- or em-dashes. (Btw, the symbol you get when you press alt gr and comma is not a dash, it's a "BOX DRAWINGS LIGHT HORIZONTAL" U+2500, for whatever reason.)