This is funny but is the opposite of the idea espoused in the correcthorsebatterystaple xkcd. Welsh words are hard for (most) humans to remember, but easy for machines to check (since wordlists are publicly available)
In the case of Welsh words, it's about as easy to remember for a non-Welsh speaker as leetspeak is, and so people will just write down the password somewhere again to remember it, which makes this less secure overall than xkcd passwords on English.
Of course a 20 random word string is less a "password" or "passphrase" and more a "passpoem". You only want one or two of those, as master passwords for a password manager.