I talked too soon. This won't work, and I have no idea why:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\setuplabeltext [en] [and-1={{, }}, and-2={{ and }}]    % Should be:  1, 2
and 3
\setuplabeltext [es] [and-1={{, }}, and-2={{ y }}]    % Should be: 1, 2 y 3
\setuplabeltext [de] [and-1={{, }}, and-2={{ und }}]  % Should be: 1, 2 und
3

\starttext
\commalistsentence[one,two,three,four] % works as expected

\language[es]
\commalistsentence[uno,dos,tres,cuatro] % takes [en] settings

\language[de]
\commalistsentence[1,2,3,4] % takes [en] settings.
\stoptext
%%%%%%%%%%%%%%%%%%%%%%%%%%

If I change \mainlanguage, the main language is used in all three cases.

-- 
Andrés Conrado Montoya
Andi Kú
andresconrado@gmail.com
http://sesentaycuatro.com
http://messier87.com
http://chiquitico.org
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Los fines no justifican los medios, porque la medida verdadera de nuestro
carácter está dada por los medios que estamos dispuestos a utilizar, no por
los fines que proclamamos.
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“You develop an instant global consciousness, a people orientation, an
intense dissatisfaction with the state of the world, and a compulsion to do
something about it. From out there on the moon, international politics look
so petty. You want to grab a politician by the scruff of the neck and drag
him a quarter of a million miles out and say, ‘Look at that, you son of a
bitch.’” — Apollo 14 astronaut Edgar Mitchell