If you wish to create your own versions of the default
error files to customize them to suit your company copy
the error/template files to another directory and point
this tag at them.
WARNING: This option will disable multi-language support
on error pages if used.
The squid developers are interested in making squid available in
a wide variety of languages. If you are making translations for a
language that Squid does not currently provide please consider
contributing your translation back to the project.
https://wiki.squid-cache.org/Translations
The squid developers working on translations are happy to supply drop-in
translated error files in exchange for any new language contributions.