For some weird reason, the brain doesn't register the word... so telling someone to relax only makes them more tense.
Maybe you should say 'Ontspan u'.
They will probably look at you with a strange face, wondering what the hell you just said.
This will get their mind of whatever they are struggling with.
Hence, they will be relaxed.
Now, does anyone have a suggesting what word I can use, because 'Ontspan u' will give the same result as if I would say to just relax.