People can develop plugins to allow Quick Look to support more file types. You can find Apple's Quick Look documentation here.
Luckily though Timac (aka Alexandre Colucci) has already created one. You can find his Timac's Quick Look plugin for strings files here.
Just download the compiled plugin, copy it to your /Library/QuickLook folder and run the qlmanage -r command in the Terminal to refresh the Quick Look plugin cache and you should now be able to view previews of files with ".strings" extensions. He even made his plugin capable enough to read .strings files that aren't plain text; some .strings files are in a binary format.
He also supplies the source on his blog, at the same link above, which should help point you in the right direction if you wanted to create a plugin for other similar files that you want Quick Look to recognise.