The locale indicated by the lc_time_names system variable controls the language used to display day and month names and abbreviations. This variable affects the output from the DATE_FORMAT(), DAYNAME(), and MONTHNAME() functions.

lc_time_names does not affect the STR_TO_DATE() or GET_FORMAT() function.

The lc_time_names value does not affect the result from FORMAT(), but this function takes an optional third parameter that enables a locale to be specified to be used for the result number's decimal point, thousands separator, and grouping between separators. Permissible locale values are the same as the legal values for the lc_time_names system variable.

Locale names have language and region subtags listed by IANA (http://www.iana.org/assignments/language-subtag-registry) such as 'ja_JP' or 'pt_BR'. The default value is 'en_US' regardless of your system's locale setting, but you can set the value at server startup, or set the GLOBAL value at runtime if you have privileges sufficient to set global system variables; see Section 7.1.9.1, “System Variable Privileges”. Any client can examine the value of lc_time_names or set its SESSION value to affect the locale for its own connection.

mysql> SET NAMES 'utf8mb4';
Query OK, 0 rows affected (0.09 sec)

mysql> SELECT @@lc_time_names;
+-----------------+
| @@lc_time_names |
+-----------------+
| en_US           |
+-----------------+
1 row in set (0.00 sec)

mysql> SELECT DAYNAME('2020-01-01'), MONTHNAME('2020-01-01');
+-----------------------+-------------------------+
| DAYNAME('2020-01-01') | MONTHNAME('2020-01-01') |
+-----------------------+-------------------------+
| Wednesday             | January                 |
+-----------------------+-------------------------+
1 row in set (0.00 sec)

mysql> SELECT DATE_FORMAT('2020-01-01','%W %a %M %b');
+-----------------------------------------+
| DATE_FORMAT('2020-01-01','%W %a %M %b') |
+-----------------------------------------+
| Wednesday Wed January Jan               |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql> SET lc_time_names = 'es_MX';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @@lc_time_names;
+-----------------+
| @@lc_time_names |
+-----------------+
| es_MX           |
+-----------------+
1 row in set (0.00 sec)

mysql> SELECT DAYNAME('2020-01-01'), MONTHNAME('2020-01-01');
+-----------------------+-------------------------+
| DAYNAME('2020-01-01') | MONTHNAME('2020-01-01') |
+-----------------------+-------------------------+
| miércoles             | enero                   |
+-----------------------+-------------------------+
1 row in set (0.00 sec)

mysql> SELECT DATE_FORMAT('2020-01-01','%W %a %M %b');
+-----------------------------------------+
| DATE_FORMAT('2020-01-01','%W %a %M %b') |
+-----------------------------------------+
| miércoles mié enero ene                 |
+-----------------------------------------+
1 row in set (0.00 sec)

The day or month name for each of the affected functions is converted from utf8mb4 to the character set indicated by the character_set_connection system variable.

lc_time_names may be set to any of the following locale values. The set of locales supported by MySQL may differ from those supported by your operating system.