Swenglish is when someone mixes Swedish and English words or grammar in their speech. It's like when you're trying to speak two different languages at the same time, like when you say "Hej, can I have a glass of vatten please?" instead of "Hello, can I have a glass of water please?" It's common in Sweden because a lot of people can speak English very well, and sometimes it's easier to say something in English than to find the right word in Swedish. But it can also be confusing for people who don't speak both languages, because they might not understand what you're trying to say.