You speak Italian wonderfully! Congratulations!
'Benissimo' = be-NIS-si-mo. 'Complimenti' = com-pli-MEN-ti.
Use this to genuinely praise someone's Italian language skills. For a non-native speaker, this is one of the most motivating and appreciated compliments. Italians are often genuinely moved when foreigners make the effort to learn their language.
'Benissimo' is the superlative of 'bene' — 'very well'. 'Complimenti!' is a versatile Italian exclamation for congratulations and praise — used for achievements of any kind: 'complimenti per la promozione!' (congratulations on the promotion!).
Il tuo accento è quasi perfetto.
Your accent is almost perfect.
Specific praise for pronunciation — highly valued
Non sembri straniero/a quando parli.
You don't seem foreign when you speak.
The ultimate Italian language compliment — passing as a native
Quante lingue parli?
How many languages do you speak?
Follow-up question showing admiration for linguistic ability
Italy has one of the lowest rates of foreign language proficiency in the EU, which makes Italians especially admiring of those who learn Italian. Italians often describe their language as 'la più bella del mondo' (the most beautiful in the world) — a belief so sincere that hearing it spoken well by a foreigner genuinely moves them.