How mischievous should a deed be to be called a prank? [closed]

The example provided in the question could be considered a prank.

You may also want to consider some of prank's synonyms, like antic:

an attention-drawing, often wildly playful or funny act or action

gag:

a laugh-provoking remark or act

or escapade

a usually adventurous action that runs counter to approved or conventional conduct

Escapade is more often today used in the plural, as an example:

their escapades at the prep school became the stuff of boarding-school legend.

Prank is by far the most common in English spoken today, but the three provided examples above all seem less mischievous in nature to me.

Following the same storyline ...

There are three people, Bob, Alice and Carl. Bob and Alice speak Spanish and English, while Carl only speaks English. Normally, Bob and Carl communicate in English. One day, as a joke to tease Bob, Carl asks Alice to take his Messenger account and message Bob in fluent Spanish, so Alice writes "¡Vámonos de juerga!". Bob replies "Cuando quieras ... Alice" followed by a winking emoji. He goes on to invite both Alice and Carl to have dinner at his place. That night, Bob cooks a Spanish tortilla, but fills it with a handful of chilly peppers, knowing that neither Alice nor Carl handle well spicy food. After one small bite, Carl starts laughing, while crying and coughing, "that's a terrible prank Bob!". Bob says "hey Carl, you know that I would never pull a prank like this on you, your taste might be playing tricks" and comes back from the kitchen with another tortilla. Alice sighs "that's the one I ordered on Messenger!". They all laugh and toast with some Rioja.

I feel that prank usually describes something that is practical and "joke" would be a good term for someone messing with someone else on Messenger.

Don't let the fact that pranks can be characterized over a spectrum of qualities disuade of its appropriateness. Your example is definitely a prank.

For example you are likely to have heard the word qualified with adjectives such as "a harmless prank" or "a needlessly cruel prank."

"Practical joke" is the closest synonym I have seen, because with a prank it is critical that there be a physical/practical element and that it not be amusing solely because of the content of the words (a "joke"). Still, not all physical humour qualifies as pranks. For example the Three Stooges poking one another in the nose, pulling ears, etc. was definitely funny and definitely not a prank ("slap stick"?).