I've seen this error too often: language menus that display flags in it.
Flags symbolize nations, not languages, so they shouldn't be used in association with languages.
You can read a rationale in these articles:
Why flags should not be used to indicate language choice
Why flags do not represent languages
(You'll find more links with more reasoning inside those two).
Also: flags have different aspect ratios (they are not the same "shape"). If you want to display a flag correctly, it's always nice to learn a little vexillology. My recommendation is this:
Flags of the World