Need one more feature: input language.

Auto detection doesn't always work well, especially when translating a small passage of text. Japanese is often recognized as Chinese, and you'd never know it, because it display say what language the selected text is detected as. Similarly, Dutch could be detected as German, Spanish as Portuguese, etc.

Input language should perhaps be a priority list of languages you most likely want to translate, and have the translation engine prefer those over whatever is autodetected, if possible.