It's important to note that Flemish is a dialect of Dutch, and while it's very similar to standard Dutch, it does have some differences. For example, in Flemish, you would use "dank u" for both singular and plural, while in standard Dutch you would use "dank u" for singular and "dank u wel" for plural.
So, "dank u" is the most common and generally accepted way to say "thank you" in Flemish.