The translation extensions, ImTranslator for Opera and Google Translate for Opera share the same translation features and capabilities except ONE.
ImTranslator extension supports multiple translation providers: Google Translate, Microsoft Bing and Translator, while Google Translate extension uses Google Translate engine only.
ImTranslator for Opera
Google Translate for Opera
If you believe that the Google translation service translates better, or if the languages you frequently use are supported by the Google Translate service only, you'd better use the Google Translate extension for Opera, than ImTranslator extension.
But if you used to ImTranslator extension for Opera, and still want to use Google Translate service only, you can disable "other translators" in ImTranslator Options (clear "Enable other translators" checkbox).