If you are using Windows 10 and French, German, Italian, or Spanish is the localized language, you can now use Cortana to instantly translate words and phrases. This is due to a new feature from the Microsoft Translator team.
In a blog post, Microsoft stated:
When you are using Windows 10 in these languages and need a quick translation of a phrase, you can now just ask Cortana and you'll get your translation in an instant. You can also type the phrase into the Cortana toolbar in case you're in a place where you can't speak it out loud.
Just like the English release of instant translation last September, you can ask Cortana to translate for you to get an answer right away. Currently, for instance, you can say in English, "Hey Cortana, translate where is the nearest taxi stand in Polish" (or any supported languages) and receive the translated phrase from Microsoft Translator right within Cortana. Now, this is also possible in these localized versions of Windows 10.
On the off chance Cortana cannot provide a translation, or if it misunderstands a request, it will perform a search and then bring up a web page with a possible solution.