English convention. That's how the language is written.
On the other hand, when I was 10 a school inspector who visited our class gave us the "i before e except after c" rule and said (foolishly), "I'll bet nobody can think of a word where I comes before e after c.:"
My hand shot up and I said, "Science."
He said, "That's different." And almost 70 years later I'm still trying to work out why.
