[i18n] Improve easy/pedantic Japanese switching

This MR is not change the user experience, just brings a small improvement of our code.

For some ideograph-phobic instances, all they have to do is modifying just 2 constatnces: specialLanguageNames in interface_language_switcher.vue and messages in messages.js. There will be no conflict when merging ja.json anymore. ja_easy.json and ja_pedantic.json are named descriptively, which include no information about which is the default.