「これ落としましたよ」って英語でなんて言うの？

First you want to start off with "Excuse me." This will get the person's intention and put them at ease because a stranger is trying to get their attention.
Then, you can ask "Did you drop this?" or "Is this yours."
Or if you saw it fall from their pocket, you can say "This fell out of your pocket". "This dropped out of your pocket". or "You dropped this."

This answer is the politest and quickest way of letting someone know that they have dropped a belonging.
The phrase 'Excuse me' is a formal way of getting somebody's attention.

When you want to let someone know that they dropped something; for example, a wallet, you can say:
-Excuse me! I think you dropped this.
-Hi. You dropped your (wallet)
-Excuse me! I think your (wallet) fell out of your pocket

To let the person know they dropped something, the quickest way to say it is "you dropped this." But if you add 'excuse me' it is a polite way of getting their attention. So you could then say "excuse me. I think you dropped this"

The two sentences you see above are great ways to let someone know that they dropped something. In the second sentence you will see the verb fall. This means that someone or something moved from a higher position to a lower position. This is a word we use in our everyday conversation and it would make a great addition to your vocabulary.