The file i18n.ts was basically only a few lines that call into
scripts/i18n.ts. Instead of having the extra file it is just as good to
have the relevant code for i18n in one file. Since i18n.ts is
imported in many client components, while scripts/i18n.ts was only
imported in i18n.ts, the latter seems better to keep.
Added some more comments and translated the Japanese comments to
English.