If you select some text on a web page, right-click on it, and then choose the context menu entry that this extension has added, that text will be sent to, and only to, https://en.wiktionary.org . For example, if you perform the aforementioned action for "dog", a web request to "https://en.wiktionary.org/wiki/dog" will be made. People who work at Wiktionary may be able to see that text and some basic information about your browser, such as your IP address.

This extension makes your browser download some flag icons from https://www.countryflags.io and https://upload.wikimedia.org. So people who work for those sites may get some basic information about your browser, such as your IP address.

Other than those, this extension does not collect, use, store any other personally-identifiable information, nor share any data with/disclose any data to any other party.

Technically, it is possible for people at Wiktionary to detect that this extension has been installed by examining the client HTML using JavaScript, but I doubt that they would do such a thing.