Answering myself on Japanese, this actually works out of the box:
\setuplanguage[ja][patterns={ja}]\mainlanguage[ja]
\definefallbackfamily
[archimate]
[ss]
[Hiragino Sans]
[preset=range:japanese,
tf=style:W3,
it=style:W3,
bf=style:W5,
bi=style:W5,
force=yes]
\definefontfamily [archimate] [ss] [Optima]
\setupbodyfont[archimate]
\starttext
Some text. 他のテキスト\\
\stoptext
I have no idea why this works and if stuff is missing because more is needed than just the above. Of course the translation of italics into weights is nonsense, in Japanese that would probably mean using quite different characters (katakana?)
Am I missing something?
G