Yes, in English write "haha." Of course your English-speaking friends may enjoy your using "jaja," if you explain it. In English, the letter j can have a number of different sounds, but the natural assumption would be that it has the usual "dzh" sound as in the words "jar," "joe," "jam," etc. and that does not sound like laughter.

Before I started learning Spanish a friend of mine from Venezuela texted me "Jaja", I got confused.

It's a different way of writing exactly the same sound. So obviously there is no need to change the noise that you make when you physically laugh, it's the same. But to a native English speaker 'jajajaja' looks weird written down. People who don't speak Spanish will be confused by it and wonder what you mean. The pronunciation of the English 'j' is very different to the Spanish one so those unfamiliar with Spanish won't associate 'jajajaja' with laughter. Personally, I am always reminded of German when I see 'jajajaja' as 'ja' is the German word for laughter.

You can always use "LOL".

Yes, use "haha" when speaking English. "Haha" makes the same sound as "jaja", just make the j sound a bit softer - say "j" and blow a breathe of air at the same time.