Yay! Pirate English! The fun of people using not-their-native-language! :]

I'm not worried about this too much really. His English is better than my French.

(The above is no more than my personal opinion.)

While "soit une fonction…" can be literally translated to "Be a function…", the latter sounds like a weird pirate English variant. Also "soit une…" sounds like you want to say "Let sumEvenNumbers be a function that takes an array an returns an integer", which is a very mathy-kind of sentence. It would be fine for theorems, but you want to tell the user (your stundents) to actually write a function. I have no idea how to form an imperative sentence in French, but the English variant would be:

Write a function sumEvenNumbers that takes an array and returns the sum of the even elements.

Google translate returns

Ecrire une fonction sumEvenNumbers qui prend un tableau de nombres et retourne la somme des éléments pairs du tableau.

which sounds oddly similar to your variant, so I don't think it's too off. At least it uses the correct words.