Word Lens (by Quest Visual) is a free app for iOS and Android that will translate signs for you. Unfortunately, there’s no Japanese on the app yet (nor many other languages), but you can translate between English and Spanish, French, German, Italian, Portuguese and Russian.
Here’s an example, using the English -> Spanish setting:
And the original:
The app includes a dictionary. So if you want to translate “jefe” back into English to confirm, this is what you’ll find:
Here’s an old video demonstrating how it works. I found it to be much the same, except the translated words were always blue and underlined.
It says that there are in-app purchases, but all the languages available were free to download at the time of this writing. If they offered Japanese, too, I’d probably be willing to pay a little for it.
The app does not recognize very stylized fonts, so it may not be able to read signs or menus with fancy fonts and cursive writing.
Here are a couple of other examples. It’s a fun thing to do while taking a walk around the city. Like a treasure hunt.