Don't cache the list of supported languages in a variable.

Why?  Two reasons:

(a) the computation is cheap and is hardly (if ever) noticeable;

(b) generating the list of display names of languages from the alist
    is more flexible because it allows user to customize the variable
    `google-translate-supported-languages-alist' (e.g., introduce an
    alias for a particular language, e.g. "Deutsch" for "German") and
    avoid having to update `google-translate-supported-languages'.