According to a literal translation, "Ritenuto" means "Held back", "Ritardando" means "Delaying", an "Rallentando" means "Slowing down".
"Allargando" means "Becoming broader."
There is a general problem with the vocabulary for describing music, whatever language you use.
How would you expect a "phat" tune to sound?
Can you play in a funky way? (literally "smelly")
The other thing that I always think about is that it is common knowledge that a large proportion of the musical 'stage directions' are added at the editing stage by the publisher, so they may have little to do with the composer's intentions anyway.
I suppose you could go for total precision and mark the score:-
"Tempo:-108 BPM, SPL 92 dB@10 metres, +2dB on third beat in the bar,played 10 ms before the regular timing."
There would be more people who would understand the Italian!
Cheers
Dave