Somewhat surprisingly, <cmd>setuphead</cmd> does have some inheritance built in, but for the table of contents, all section levels must be treated individually.
Another trick to achieve the same:
<texcode>
\setuplabeltext
[sl]
[chapter={{},{.}}]
\starttext
\chapter{English}
without a dot
\mainlanguage[sl]
\chapter{Slovenian}
having one
\stoptext
</texcode>
<context>
\setuplabeltext
[sl]
[chapter={{},{.}}]
\setuphead
[chapter]
[style=bold,before=\smallskip,after=,page=no]
\starttext
\chapter{English}
without a dot
\mainlanguage[sl]
\chapter{Slovenian}
having one
\stoptext
</context>