I don't remember the details.. but..
Database locale never change... so, your ODBC database locale must be en_US.UTF8
And need exists a conversion code-set file to this work. To check this you need look in the $INFORMIXDIR/gls/cv* directory . Look for *.cv files.
Will need to exists the conversion from en_US.UTF8 to de_DE.819 and vice-versa .
Looking for this conversion code-set on the IDS 11.50 UC1DE , I found the file e01c0333.cvo and 0333e01c.cvo , they are the respective conversion..