I get that dynamic translation of the body text on a static website is too much to ask, but is there an easy way to at least localize the site navigation and dates and times, e.g. with a source file containing the translated terms and date formatting?
But that might not be what you want. Translating themes is also possible. The basic idea is to create a custom theme, find the things you want to localize and make the required changes. Hugo comes with tools to translate content using separate language files and support multilanguage sites. But they might be overkill in your case; you could just hardcode the translations.
Now, if you would call it easy or not varies from person to person. Someone used to Hugo templates will find it easy. On the other hand, a person without previous web templating experience will probably find it pretty hard.