X Code 12 compiles my app. But X Code 3 says that all FindPanel and InfoPlist strings have invalid content. Approximately 12 localized strings for each, and all fail. Other string files are accepted. No obvious problems in any of the files, which work fine on X Code 12.
I solved this problem by looking at the bad string files with hexdump. They were UTF-8 files and the English versions only had ascii. So I expected hexdump to show something like 20 3f 5a 45 ..., but actually it showed 00 20 00 3f 00 5a 00 45 .... In earlier XCodes, this gave a warning, but now it is an error.