Is There Proof that “Vader” was Changed to “Fener” Because Vader Sounds Like “Toilet”?
In Italian “vaso” means “bowl” and can refer to “toilet bowel”, likewise “water” (pronounced “vater”) has the same connotation. This is likely the reason Darth Vader’s named was changed from Lord Vader to Lord Fener in the Italian version of Star Wars due to this.
As an Italian speaking reader confirmed, “Lord Vader” would have sounded “extremely ridiculous”.
Although the official reason for the name change can’t be confirmed, the translation itself is easily confirmed (see the Italian Wiki page on “water”). 
Thus, it is very likely this was the reason for the name change. So despite any official proof, the claim can be characterized as a fact.
Did Other Star Wars Characters Get Name Changes?
Other Star Wars characters also had their names changed (for example Jabba the Hutt becomes Jabba le Forestier, Jabba the forester, in French), and of course translating the movie meant translating famous lines like [spoiler alert], “Luke, I am your Father” (as illustrated by the video below).
Star Wars – I am Your Father – (Multi-Langage) 20 Langues.
FACT: Darth Vader’s name was changed in more than just the Italian version, although suspected reasons for the change differs with each translation. Some speculate Vadar sounds too much like “father” in some languages (although its debatable as “Darth” essentially means “Dark” in the Star Wars Universe, and is translated this way in the French translation for example, and “Vader”, “Fener”, and other names all imply the concept of “Father”, thus the general translation of “Darth Vader” as a concept is “Dark Father”; i.e. a Sith who is Luke’s father), but others speculate naming was changed simply to “roll off the tongue” more easily in a given language. Upon research it seems a bit of both is true for any translation. 
Uh no it is a fact! In Italian we do use the word WATER (pronounced the Italian way, thus with the first W sounding like a V) to refer to the toilet. Vader would have indeed sound extremely ridiculous