There is in spanish a word "pasadomañana" to refer to the day after tomorrow, I wonder if maybe there is an popular/informal way to mean the same without say the day after tomorrow
Yes absolutely! One of the most popular expressions to refer to the day after tomorrow is "in two days", an expression slightly shorter than "pasadomañana". Or something similar, depending on the context.
(In fact, it's so common that any answer that dares suggest it might seem condescending, but I'll try my best.)
This expression can be used in pretty much any type of writing: formal, informal, headlines, txtspk, spoken, written, period-specific, etc.
(This is not something true for "overmorrow", a now-obsolete word which appears neither in COCA nor Google N-Grams. It was never popular, since the OED marks it as "obsolete rare", so if you use this word you risk clarity.)
When used in informal writing, the number isn't always written out: "in 2 days". More rarely, in extremely informal writing, sometimes the space after 2 is removed. In places where hashtags are common, you can sometimes see things written as a hashtag, although this is also pretty rare: "in #2days".
There is also an adjective form, seen for example on Amazon:
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In recent years, casual conversation has begun using "sleeps" to describe future days.
Just two sleeps until I get my new Elmo slippers!
See this Twitter search about "two sleeps".
Alternatively, as shown by Tony's character in Seinfeld, you could just use the Spanish (pasadomañana, in this case) to add flavour to what you're saying.
Not really an expression, but how about this one: overmorrow?