Both are correct. There is just a very small difference (but even native speaker don't care because the difference is unneeded). "Ich bin froh, dass DAS ein tschechisches Wort ist." "das" means something specific. Like something that's standing right in front of your nose. "Ich bin froh, dass ES ein tschechisches Wort ist." When you say "es" it could be every sentence that you said befor. But actually it's clear what you mean with "es". But these are very detailed things, so you can use both in any situation with the exactly meaning.

Both are ok depending on context.