I recently managed to spill coffee into the keyboard of my laptop. Because replacing it was horribly expensive I've started using a cheap silicone keyboard skin, a protective cover that fits over the keys. It gives the keys a slightly dead feel, but I can still type well on it. It can be put on or peeled off in seconds.
The one I got was clear, but you can also buy them coloured and pre-printed for various keyboard layouts including American and European layouts, Russian, Hebrew, Japanese, Taiwanese Chinese, etc. They're very light and thin, and it would be easy for a translator or other linguist to carry several around in plastic wallets in a laptop case, and put on whichever one is needed. It's a lot easier than using individual labels for each key, especially if you are working with several alphabets, and a lot lighter than carrying a plug-in keyboard.
As an example of what is available, here are some I found for various Apple laptops in the UK. Availability in other countries may be very different, of course.
http://www.ebay.co.uk/itm/271265787839 - Clear, UK and US layouts in various colours - £2.69 (UK supplier)
http://www.ebay.co.uk/itm/381407621720 - Russian/UK & EU - £0.99 (ships from Hong Kong)
http://www.ebay.co.uk/itm/221889442597 - Japanese - £1.29 (ships from Hong Kong)
http://www.ebay.co.uk/itm/181796801300 - Taiwanese - £4.99 (Ships from China)
http://www.ebay.co.uk/itm/181965879813 - Arabic / Russian / French / Portuguese / Spanish / Hebrew / Italian / English versions - £5.99 (Ships from China)
I don't think the range is so extensive for other laptops - Apple have the advantage of keeping the same standardised layout on a lot of laptops over a decade or more.
I think you could also start with one of these skins and add labels to it for other languages - again, a lot less messy than labelling the keys directly - but I don't know how well labels will stick to them.