When you want to change the quotation marks it’s better to change them with \setuplanguage
and set the right sign for the current level with \symbol[…quotation] and \symbol[…quote].
\setuplanguage
[de]
[ leftquotation=»,
rightquotation=«,
leftquote=›,
rightquote=‹]
\mainlanguage[de]
\starttext
text \quotation{text} text \quote{text} text
text \quotation{text \quote{text} text} text
text \quotation{text \quotation{text} text} text
\setupdelimitedtext[quotation:1][left={\symbol[leftquotation]},right={\symbol[rightquotation]}]
\setupdelimitedtext[quotation:2][left={\symbol[leftquote]}, right={\symbol[rightquote]}]
text \quotation{text \quotation{text} text} text
\stoptext
BTW: There is a change in the way how to set the values for a certain delimited text level in MkIV as shown above.