I was told years ago to fill it with warm water and put a tablespoon of washing powder in it.
Soak overnight then gently scrub (I know, not two words that go together) with a bottle brush.
These days, the microfibre foam washing up brushes would be ideal I would think
If the glass on the inside has turned cloudy, it may never again appear to be clean, since whatever was in the jug may have etched it. I've had antique crystal decanters like that.