今お手洗い誰か入ってったよ！って英語でなんて言うの？

If someone is already in the toilet, it is engaged
If no one is in the toilet, it is vacant

Public toilets in England usually have a sign on them which says either Engaged or Vacant

1. typically, after making this comment, your friend would ask 'why?' Then you would explain that you saw someone going in the restroom: "There's someone in there already."
2. To cross one's legs has the usual meaning: Place one leg over the other while seated.
   ‘I crossed my legs and leaned back in my chair.’
   However, in the context of waiting for the restroom, this means that you may have some moments of discomfort as you wait for the occupied restroom to be vacated. Typically children cross their legs while standing. This is a humorous comment.

The sentences above means that you are confident someone is in the restroom since you saw someone entering.

When talking about a toilet or bathroom we often use the word restroom as a more polite way of saying this
if someone else is in the bathroom already then you can simply say 'somebody else is in the bathrom'
occupied also means that something is already being used by someone

"Someone just went in there before you" 'went in there' can be used describe a certain location, in this case a toilet.
"The toilet is occupied" 'occupied is often used in more formal and polite terms, used to describe something that is in use.
both well known and frequently used sentences.