"I am eating an apple."
It's not a "t." It's an "l." There are two "l"'s in the Polish alphabet. The first, "l;" pronounced "leh," is used like we would use it in the English language. The second, pronounced "weh," has a slash that runs ascending, diagonally from left to right; and is used where and how we would use a "w."
I'm afraid that "Jestem Polski" doesn't make sense. We can say "polski samolot" (a Polish airplane), or "polski piłkarz" (a Polish footballer), but to say what is my nationality, we use nouns. A man will say "Jestem Polakiem" and a woman will say "Jestem Polką". They are capitalized, while the adjectives are not.
Somewhere in the settings on your phone you can enable a Polish keyboard, but an easier solution may be downloading an app call SwiftKey and then you can have up to 5 keyboards there. For example you could have the diactritic marks for German, Spanish and Polish on the same keyboard.