[Answer from August, 2017. Compiler may have changed]
Search for a syntax error in your .strings File
I had the following line:
"Changed your mind?" = "¿Cambiaste de opinión?";,
And somehow that made the compilation succeed, even the command plutil -lint Localizable.strings said it was okay. After I took the same comma down from each file in that line, now the compiler is able to detect there are errors.
The problem before was the compilation succeeded, but because of the comma the .strings file was actually wrong so nothing after the comma was read. I don't know if the comma has any use in .strings files.
Note:
plutil -lint Localizable.strings is used on the Terminal tu debug .strings files as Xcode cannot tell where the .strings compilation wen wrong.