The easiest is for domains .at, .hu, .hr, because we know for sure they are from Austria, Hungary, Croatia.
Twitter users usually have their location set up.
If they don't, then we also look for "hints" inside the post.
For example, someone mentions "Wien" so we recognize it is Austria. If someone writes about "Ban Jelačić Square" we recognize it is Zagreb, Croatia.
If there isn't any data, the tool looks at the language.