I have found a lot of old information about Debian webalizer and stats language, saying the only way to change language is compiling from sources. Even it's an entry on FAQ:
I have researched a little about it, and found that you don't need to compile webalizer for generating stats on your language
Debian webalizer uses gettext, so it will generate statistics on the default locale of your system.
With Debian > 4.0 (etch), you can change your locale with:
You can momentarily force generation in another language simply changing your LANG env var:
export LANG=es_ES.UTF-8; webalizer -c xxxx
I'm sure Debian's stock webalizer + ISPConfig2 are generating statistics in spanish on my server, only having spanish as default system's locale.
I'm not sure about ISPConfig3. Will try it this night.
P.S. Please, edit or delete that entry on FAQ, it's guiding users by a hard and no necessary way