From the question and your comments it seems you have around 400 strings only 20 of which should not be localised. With that ratio, as you yourself say, changing them all and then undoing the change for 20 can make sense.
To do this get TextWrangler, or BBEdit, and perform a multi-file pattern matching search and replace. You can confine the search to files ending in .m
or .h
. The task will be quick and easy, apart from those 20...
HTH