Basically, QGIS supports internationalization, but QGIS 1.8.0 Lisboa has some problems with correct encoding of shapefiles. There are some workarounds given here:

How to read Greek fonts (ISO-8859-7) in shapefile attributes within QGIS 1.8.0?

which are a bit Operating-system-dependent.

The matter was fixed in QGIS Master, but currently the bugfixing for the upcoming QGIS 2.0 may lead to a unstable QGIS Master version.

So you might better wait until QGIS 2.0 is published.

EDIT: This only affects shapefiles, other data sources should work correct.

Basically, QGIS supports internationalization, but QGIS 1.8.0 Lisboa has some problems with correct encoding of shapefiles. There are some workarounds given here:

How to read Greek fonts (ISO-8859-7) in shapefile attributes within QGIS 1.8.0?

which are a bit Operating-system-dependent.

The matter was fixed in QGIS Master, but currently the bugfixing for the upcoming QGIS 2.0 may lead to a unstable QGIS Master version.

So you might better wait until QGIS 2.0 is published.

EDIT: This only affects shapefiles, other data sources should work correct.