Is there any way to switch keyboard layout/language when you're using an external bluetooth keyboard that isn't made specifically for iPad (i.e. has none of the "special" keys)? Perhaps some combination of keys pressed at the same time?
I only thought of a very impractical workaround - disconnecting the BT keyboard, waiting for the on-screen keyboard to appear, switching the layout there, connecting the BT keyboard back again.

Settings > General > Keyboard > Keyboards, then select the language.

There are two sets of choices.

The first is for the Software (touch) keyboard. This is new with iOS 6. I'm glad your question made me look, because I hadn't noticed it yet.

The second is for Hardware (bluetooth) keyboards.

And that's pretty much all there is; so I hope it's what you needed.

Not really. I probably should have used "while" instead of "when"... I meant switching layouts while typing (e.g. if you want to use another keyboard layout just momentarily to enter some characters from that layout or emoticons from Emoji layout, and then switch back to the main one).

I'm still on 5.1.1 for now, so the Settings part is a little different there, but in any case your suggestion would be even more "complicated" than my workaround.
In the Settings, I go to General > International Keyboards > and there's the list of installed keyboard layouts between which you can choose while typing.

So if it's the on-screen keyboard, there's the globe icon to the left of the space-bar. Pressing it switches to the next layout (and for a second, the layout's name is displayed on the space-bar briefly).
When you're using an external ("generic", not iPad) Bluetooth keyboard without any special keys, there seems to be no way to quickly switch to another layout (as the "globe" key does not exist on such hardware keyboard, and the on-screen keyboard is not displayed while the hardware BT keyboard is in use).
My current workaround is: Switch off the BT keyboard; wait a few seconds for the on-screen keyboard to appear; press the globe "key" there to switch to the desired layout; switch the BT keyboard back on, and wait a few seconds until it's re-connected. A bit lengthy, though.

So, I'm guessing the only option -- if anyone was able to discover it somehow -- would either be a magic combination of keys to be pressed (on the BT hardware keyboard) in order to switch to the next layout, or perhaps some way to force the on-screen keyboard be displayed despite an external BT keyboard is connected and in use. Is at least the latter possible somehow?

My keyboard dissapeared how do I get it back

Originally Posted by Commodore

So, I'm guessing the only option -- if anyone was able to discover it somehow -- would either be a magic combination of keys to be pressed (on the BT hardware keyboard) in order to switch to the next layout, or perhaps some way to force the on-screen keyboard be displayed despite an external BT keyboard is connected and in use. Is at least the latter possible somehow?

Sorry. I don't know of any magic keyboard combination keypresses that would help.

On the Apple bluetooth keyboard you can turn the touch keyboard on/off by using the CD eject key. If your Bluetooth keyboard has an eject key give it a try. It only works when you're in an editable text field.