There are Safari Extensions available for page or inline language translation that use either Google or Microsoft translation services. Translate seems to offer the best experience. Franker requires one to sign-up for a capped free Microsoft Translator account or a paid Google Translate account. I didn't sign-up to either service to try the extension further. TranslateMe works as advertised, but exhibits one unexpected behaviour: jumping to Window 1 when clicking the translation button from another window. TranslateMe does offer the ability to listen to the original language be spoken, which appears to work well. All three extensions are free, so it may be worth trying them all.
Note: Many of the Japanese websites offer English language versions.
Translate extension in use using Google Translate:

Translate extension in use using Microsoft Translator:

Safari's Extensions pane showing the Translate extension:

Franker's help message:

TranslateMe's popup translation window:
