[NTG-context] index in swedish doc (mkiv)

```Hi all,
I want to have an index in a swedish document, but when I add it I get
```
%%%%
\mainlanguage[sv]
\starttext
Test\index{test}
\placeindex
\stoptext
%%%%

Just uncomment the \mainlanguage[sv] and you will get a "t" instead of
"unknown" as expected.

If there is something that has to be added when it comes to swedish
indices I am willing to do that.

By the way, this problem does only appear in mkiv, in mkii it works as
it should.