When I try to use https://tools.wmflabs.org/isa/campaigns/56/participate?country=Italy , because the interface is in English, only English content is shown (mostly translated templates). This makes the task impossible.

While it would be nice to have an interface translation for all languages, that's not enough to cover all use cases. Users must be able to choose their own content language, even if they are using another interface language. See https://www.mediawiki.org/wiki/Manual:Language for the distinction.

An easy way to do this is to have another language selector for the content language, as we currently do elsewhere. Alternatively, content in all languages could be shown. More complex solutions would involve some automatic selection of the preferred content language and a system of fallbacks, perhaps using jquery.uls: these can be implemented later (I see there is a task for language selection).