There are definitions in jdk/src/java.base/unix/native/libjava/locale_str.h:

---
138#ifdef MACOSX
139 "sr-Latn", "sr_CS", // Mappings as done by old Apple JRS code
140 "tk", "tk-Cyrl",
141 "tt-Latn", "tt-Cyrl",
142 "uz", "uz_UZ",
143 "uz-Arab", "uz_UZ",
144 "uz-Latn", "uz_UZ",
145 "zh-Hans", "zh_CN",
146 "zh-Hant", "zh_TW",
147#endif
---

With the fix to JDK-8160199, locale strings generated from getMacOSXLocale() function are now all in valid language_country form, those mapping definitions are no longer needed.