You'll need to activate the mode (by saying something like "turn on interpreter mode or "Help me speak Spanish") in English, French, German, Italian, Japanese or Spanish. Once you've started it though, the interpreter will translate between 26 languages, with support for more on the way.
When we tried the mode at CES, we found it to be slow and stilted, which could cause complications for more complex translations. But, as we've seen with Google Translate over the last decade or so, the interpreter will likely improve. In any case, the rollout is another useful step towards us busting through language barriers with greater ease.