Are there any interpreters available for the Android platform for any language at all? If so, do they work with both physical and on-screen keyboards?
I think you can use android scripting engine and play with python and other languages on the device. An article¹ seems to confirm that (with a screenshot of a huge "Edit" button ;-))
¹ http://www.linux-mag.com/id/7370/
You can use Frink. A "simple" oo language which is mostly used as "calculator" with units or unit conversions. See: http://futureboy.us/frinkdocs/android.html
RFO BASIC! is availble. It is a very full featured basic and can even compile into stand alone code. Availble for free in Android market
http://www.laughton.com/basic/