I think this is very bizarre because many dictionaries translate "Wichser" into "wanker" in English, and into "enculé" in French, when "wanker" and "enculé" don't mean the same thing at all...
"wanker" means someone who masturbates, when "enculé" means someone being sodomised. I can't see the link at all...

A German "Wichser" is also someone who masturbates and it has the same rude insulting meaning as in English. I don't know about French. I guess, because "Wichser" and "wanker" have two meanings – a literal (masturbator) and a non-literal (just an insult) – someone translated just the latter into French. Does "enculé" have the insult meaning as well?

Ah, you translated it that way. Okay then. If "enculé" can be used as an insult in the same way "Wichser" can, I see no problem here.
Maybe French implies someone willingly being sodomised?

Yes, "enculé" is also an insult in French. But it's quite different from "branleur"/="[eng]"wanker"

[fra]"branleur"/="[eng]"wanker" implies that the person being insulted does nothing / idles / is lazy...

[fra] "enculé" as an insult means the person is wicked, mean, nasty...

Interesting.
I don't know about "wanker" in English, but
[de] "Wichser" doesn't imply idleness, but rather implies a person who did something rude, extremely offensive or caused severe damage to someone, like breaking something on purpose, killing someone's dog, spilling a drink on someone... a wicked, mean or nasty person. So semantically "enculé" and "Wichser" seems REALLY close.

I'm not sure who might be called a wanker, though.

Yes, so this confirms my feeling that dictionaries are wrong to translate "Wichser" into "wanker" because that's different...
Probably puritan scholars didn't dwell much into the subject...

The french "lighter" version of "Wichser" is "enfoiré", which means the same thing but is more ironic, or 2d degree, not as rude. What would be the German equivalent?

Well, "Wichser" > "wanker" then seems to be a translation of meaning (1) of the word "Wichser", the literal meaning. While the French "enculé" is a translation of meaning (2), the non-literal insult. A good dictionary should include both.

So, is "enfoiré" an ironic, less-rude version of "Wichser (1)" (masturbator) or "Wichser (2)" (mean person)? I'll have to think about it, as I hardy ever use such kinds of words. :)