Simple questions for my .jp applications.
 

I am developing a mobile app for katakana learning purpose and i don't know how to say " I am learning katakana " .
It's : Katakana o naratteimasu?
And how it's written? katakana ==カタカナ , o naratteimasu [hiragana/katakana] ?

A-ri-ga-to-u ありがとう !

「カタカナが勉強します。」 is good.
Either that or 「カタカナが習いって。」

There are at least two mistakes in each of these sentences.

It is written like this: カタカナを習っています。

I'm afraid that packetpirate's sentences are not correct, although it was a good try. The second one has the wrong particle and is an order rather than a statement, and the first one also has the wrong particle and is in the future tense rather than present.　『かたかなを勉強しています』 would probably be best (it's more natural to use benkyou shiteimasu rather than naratteimasu in this situation).

I have written katakana in hiragana (かたかな) above, but you could also write it in katakana (カタカナ) or kanji (片仮名) if you wanted. Actually writing katakana in katakana seem to be the most common these days.

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I will use RickOShay's version. I think(I read on the internet) that sentence "katakana benkyou shiteimasu" is used when i want to tell someone: I study katakana.

Am I right? Thank you guys.

Go ahead and use RickOShay's version.

In Japanese, you simply don't "study" katakana. Katakana is something that can be learnt in a few days. Same thing with the Roman aphabet. Simple things that you can learn in a short amount of time without even a teacher, you don't 勉強する. You can only 習う or 覚える　them.

You can 勉強する chemistry, math, sociology, philosophy, Italian, Japanese, etc. It takes years to study those but not the alphabet. You can only 習う or 覚える the Roman alphabet, Japanese hiragana or katakana. Do you see the difference?

What is really funny is the words kind of have the exact same nuance in English.

I really don't think so. In English it's perfectly acceptable to my ear to say "I'm studying katakana [right now]." But you would only have a two-week window in which you could say that before sounding dumb or lazy (or, to be fair, very busy).

Really, six months later you're still studying katakana?.

To my native ear, "study" carries some level of effort, and if you're literally in the middle of learning something that takes more than a few minutes to comprehend, you can use "study" while you're learning that thing.

You would never "study" the names of three flowers, but you could "study" the scientific names of a hundred flowers.

But I think reasonable native ears could differ on this. It's borderline.

Look at this phrase: John studied his cheating wife's face. That is a short-term activity, but implies some effort on John's part.