Since Australia uses the same alphabet, and is closely related to the UK in many ways, it would be a technically easier market, and one to establish in a lot quicker. Not trying to pump Australia, but that's a far-removed English speaking country that Roku isn't in. US, UK, Canada, and Ireland are already there. Germany would be an easier market than Russia or China, from both the culture and technical standpoint, but because of the language differences, not as easy as Australia or other English-speaking countries.
I suspect Roku is working harder to expand offerings in existing countries than expand into other countries. Again, limited resources is a major factor, as well as return on investment and speed of return (in today's climate). Anything that uses a vastly different alphabet is a technical challenge. Roku is smart to spend resources where they are, and enter new markets with a well-thought plan (learning mistakes from the UK launch).
Basil
Longtime Roku user