To enable teletext decoding you must specify the VBI device to get teletext data from (usually /dev/vbi0 for Linux). This can be done by specifying tdevice in your configuration file, like shown below:
tv=tdevice=/dev/vbi0
You might need to specify the teletext language code for your country. To list all available country codes use
tv=tdevice=/dev/vbi0:tlang=-1
Here is an example for Russian:
tv=tdevice=/dev/vbi0:tlang=33