All the scores I studied as a piano student used Italian terms for performance instruction, or their abbreviations: "rit." and "accel." etc.
I'm wondering - for my own compositions - whether I should uphold that tradition, or use English equivalents. It seems odd to write "molto vivace" in a score, when I mean "very lively". I know just enough phrasebook Italian to say "piu rubato", but I'd slip into English to write something like "with emphasis on the melody".
In scores I've produced already, I have used Italian instructions, like "ritardando". I'm thinking of editing them to use the word "slower" instead, and to use all-English annotations from now on.