VOCALOID2 Editor's GUI text will be displayed in the language of the VOCALOID2 product that you installed the last.

For example, if you install SWEET ANN (English VOCALOID2) on the computer which HATSUNE MIKU is already installed, the language of VOCALOID2 Editor GUI is switched to English. Likewise, if you install HATSUNE MIKU (Japanese VOCALOID2) on the computer which SWEET ANN is already installed, the language of VOCALOID2 Editor GUI is switched to Japanese.

When you want to display GUI back in English after installing Japanese VOCALOID2, just install English version of VOCALOID2 Editor again. (It is not necessary to re-install VoiceDB and ExpressionDB.)