[HACKERS] initdb initalization failure for collation "ja_JP"

Building on Cygwin latest 10  beta1 or head sourece,
make check fails as:

---------------------initdb.log -----------------------------
The database cluster will be initialized with locales  COLLATE:  en_US.UTF-8  CTYPE:    en_US.UTF-8  MESSAGES: C
MONETARY:en_US.UTF-8  NUMERIC:  en_US.UTF-8  TIME:     en_US.UTF-8
 
The default database encoding has accordingly been set to "UTF8".
The default text search configuration will be set to "english".

Data page checksums are disabled.

creating directory 
/cygdrive/e/cyg_pub/devel/postgresql/prova/postgresql-10.0-0.1.x86_64/build/src/test/regress/./tmp_check/data 
... ok
creating subdirectories ... ok
selecting default max_connections ... 30
selecting default shared_buffers ... 128MB
selecting dynamic shared memory implementation ... sysv
creating configuration files ... ok
running bootstrap script ... ok
performing post-bootstrap initialization ... 2017-05-31 23:23:22.214 
CEST [16860] FATAL:  collation "ja_JP" for encoding "EUC_JP" already exists
-------------------------------------

This does not happen on 9.6.x.
Any idea what to look ?

Regards
Marco