Fulltext index

Hello again,

my previous post wasn't answered. I think i told my question wrong :-).

I have a table like
a    | b
--------------------
'de' | Hallo welt
'en' | Hello world

How can I create a full text index over b?
CREATE INDEX idx ON table USING gin(
to_tsvector(
    case
        when a = 'de' then 'german'
        when a = 'en' then 'english'
        else 'english'
    end
), b);

This doesn't work. Error Message in german:
FEHLER: Zugriffsmethode »gin« unterstützt keine mehrspaltigen Indexe
SQL state: 0A000
means, gin doesn't accept multicolumn indexes.

Any ideas?

Greetings Andreas
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