When you select the Use Base Internationalization option in the project editor, Xcode generates strings files for each your project’s user interface documents. 11462724
To resynchronize your strings files with new content from your user interface documents, use the --generate-strings-file option of the ibtool command to produce new strings files. Then, manually merge the new files into your existing localized strings.
I read these release notes before, looking for a solution, and tried to re-generate the whole string file, because I thought it was somehow corrupted, but:
the first string files were generated at the end of the developing process. The segmented controls were not added after this;
Files re-generation gave the same result;
The segmented control titles are perfectly listed into the string file.
The problem is strictly limited to NSSegmentedControls. Every other control is correctly translated.
OK, I have reported this as a bug under Problem ID: 12724334