Ting-Wei Lan authored Apr 29, 2018 and Philip Withnall committed May 01, 2018

It seems that the test expects g_date_time_format to return formatted
results in English, and there is no setlocale (LC_ALL, "") call in the
file so the test does run in the default C locale. However, gettext
seems to read the value of LC_MESSAGES from the environment by itself.
Even if the value of LC_MESSAGES locale is C because of not calling
setlocale, gettext still translates the name of the month according to
the LC_MESSAGES environment variable, causing g_date_time_format_locale
to fail on the "%b" test case because it cannot convert UTF-8 text
returned by get_month_name_with_day to ASCII.

To avoid the test failure, we set the LC_MESSAGES environment variable
to C before format tests and restore it at the end of the function.

https://bugzilla.gnome.org/show_bug.cgi?id=795165