The letter in the middle of "jabłko" is not T. It's Ł, a special variant of L, very common in Polish. It makes the English 'w' sound. ł looks similar to t in lowercase, but it's a completely different letter.
"jabłek" would be plural Genitive.
The Nominative (the basic variant) of "apples" is "jabłka". For not masculine-personal nouns (so most plural nouns), the Accusative is identical to Nominative. As you use Accusative here, you have "Oni sprzedają jabłka".
Well, if anything, use L. Well, it depends on what you use. And what is your first language. If you normally use an English keyboard, you can just use a Polish one instead and remove the English one - Polish has everything that you need for English. If you use a smartphone app, then I would recommend using SwiftKey.