This joke makes fun of the parity between ‘space’, as in the invisible character between words, and ‘space’, as in the void between astronomical bodies. In this case, it is said that the word ‘space’ was never meant to be there at all, but it was included as a word due to a formatting error.

Oooohhhh, now I get it!

This took me far longer than I’m willing to admit

The title text is factually wrong. The sign actually reads: “Welcome space to space the space International Space Station Exclamation Point!”

According to Wiktionary, Russian uses different words (as do a lot of languages for that matter) for the two concepts, so it’s hard to imagine how this could have happened.

Yes, I know it’s a joke. I think it would have been a cleverer joke if Russian was a language that used the same word for both, like English.

But then, if you do find a language that does this, the word order is generally different, and the word is generally conjugated into an adjective so it still can’t be mistaken for a noun. (This is based on what happens with “European Space Agency” which would otherwise be a better candidate for the joke.)