The Dutch word that immediately comes into mind is the xylofoon. It sounds similar to the English xylophone. On the internet I found this French stamp, showing one:

The neXt words are similar in Dutch and many other languages:
On the internet I searched for stamps about the historical figure Xerxes (I or II). But the only one directly showing 'xerxes' was this stamp of this ant whose second name is xerxes:

Years ago I saw a nice movie (one of my favourites), located in the south of Africa, whose main character was named Xi. I thought them to speak Xhosa, but I was wrong, though the language might have some similarities (I should study on that to be sure). Despite of this, 'Xhosa', both language and people, of course offer a new chance to find stamps. And I found stamps dedicated to the Xhosa people, among them this one:

See what other stamps related to the letter 'x' mail lovers have found on and via today's Sunday Stamps!