I'm Italian, so I use italian language on my tablet (note 10.1), but I have also a bluetooth keyboard with english layout that I use to write little snippets of programmes, the problem is that I don't know how to set english language only for the keyboard. Can someone help me to solve this problem?
You need to install physical keyboard layouts. One of the best extensions to do that is this:
It is free, and does nothing else. You use the standard settings page to select the hardware keyboard layout.
(It is mystery why Google does not provide the layout files by default.)
The best way to use two different languages either on a smartphone or on a tablet is from device standard settings, with device keyboard and also with external USB and bluetooth keyboard. To use more than one language follow the images: [I can't post images yet]
General Settings -> Language and input -> Option of Samsung keyboard -> Input languages
Here you can chose between many languages. So you have setted correctly. Now to use one or the other language when you write is very easy, you have to slide with finger on the spacebar of the device keyboard.
You might wish to give the External Keyboard Helper a try. Take a look at this screenshot (click for large variant):
External Keyboard Helper (source: Google Play; click images for larger variant)
That's just two of its setting screens. See the second entry in the first? Chose keyboard layout. That's what you are looking for. And it should only apply to the external keyboard, while leaving your "software keyboard" untouched.
Let me suggest you Russian Keyboard app. It allows you to choose different layouts for hardware and software keyboards (one or multiple). It supports any layout (including your own). And it is free.
The only thing you should know is that selected layouts work as long as the app is selected as a current input method.