With thanks for a suggestion from an anonymous user, WP Translator's interface has been simplified. With version 1.1, you choose default origin and target languages the first time you run it, or when you press the Option key. Thereafter, WP Translator just translates selected text without showing you any language lists.
If you press the Command key when running WP Translator, it shows you origin and target lists with the default languages switched. You can click OK to accept these choices or choose any other languages from the 72 available. These choices apply to the current translation only, and do not reset defaults.
Free in the Files section here, as "Translator 1.1.sit". The distribution includes Translator 1.0 for those who prefer that interface.