A mistranslation has to be explained in order to be funny or even understood. There has to be context in a picture in order to make it interesting. In this case, the translator may have misheard or misread the handwriting, or generally misunderstood the original.
There is no other explanation. Listening to the phrase “cyclist dismount” cannot possibly yield a translation to Welsh of “cystitis retards” as even a handwritten note of the word is a stretch. No amount of deduction could have such a disastrous result. Even if it were a native English speaker who were to translate it to Welsh, there is still no reason to write down “llid y bledren dymchwelyd,” when the nearer translation would have been “beicwyr” for cyclist, and “oddi ar y beic” or “disgynnen” for dismount or fall from something. A misunderstanding of the original phrase will always cause a wrong translation.
When translating a foreign language, this will occur quite often, unless the sign is explained properly. Listening over the phone, or writing it down by hand does not guarantee the acceptable results.
However, the translator does have to understand what he is writing. A Welsh speaker could not have possibly written “cystitis retards” on a road sign. As Wales is a part of the United Kingdom, foreigners would have assumed that this could not have happened. Unfortunately, it does. For Americans, the difference is like blindly asking someone to translate English to Spanish, or even Cajun. In both cases, it cannot be assumed that they can understand one another.
It is to be expected that most Welshmen, if not all, know the English language, although it cannot be presumed that an English speaker would know Welsh. No matter who wrote the sign, it all boils down to a lack of understanding, and not asking about it. Any person given a task should ask the right questions in order to do the job. In this case, the person who wrote the sign did not bother to research, or asked a native Welshman, or have it checked afterwards.
This is a road sign, and it is not funny when mistranslations such as these happen.