Double check that you have the dir /usr/share/mariadb/
❯ ls -l /usr/share/mariadb/
total 2936
drwxr-xr-x 2 root root 4096 nov 20 23:42 bulgarian
drwxr-xr-x 2 root root 4096 nov 20 23:42 charsets
drwxr-xr-x 2 root root 4096 nov 20 23:42 chinese
drwxr-xr-x 2 root root 4096 nov 20 23:42 czech
drwxr-xr-x 2 root root 4096 nov 20 23:42 danish
-rwxr-xr-x 1 root root 3527 oct 30 15:38 debian-start.inc.sh
drwxr-xr-x 2 root root 4096 nov 20 23:42 dutch
-rwxr-xr-x 1 root root 27 oct 30 15:38 echo_stderr
drwxr-xr-x 2 root root 4096 nov 20 23:42 english
drwxr-xr-x 2 root root 4096 nov 20 23:42 estonian
-rw-r--r-- 1 root root 2165262 oct 30 15:38 fill_help_tables.sql
drwxr-xr-x 2 root root 4096 nov 20 23:42 french
drwxr-xr-x 2 root root 4096 nov 20 23:42 georgian
drwxr-xr-x 2 root root 4096 nov 20 23:42 german
drwxr-xr-x 2 root root 4096 nov 20 23:42 greek
drwxr-xr-x 2 root root 4096 nov 20 23:42 hindi
drwxr-xr-x 2 root root 4096 nov 20 23:42 hungarian
drwxr-xr-x 2 root root 4096 nov 20 23:42 italian
drwxr-xr-x 2 root root 4096 nov 20 23:42 japanese
drwxr-xr-x 2 root root 4096 nov 20 23:42 korean
-rw-r--r-- 1 root root 1683 oct 30 16:09 maria_add_gis_sp_bootstrap.sql
-rw-r--r-- 1 root root 1958 oct 30 15:38 mariadb_performance_tables.sql
-rw-r--r-- 1 root root 623733 oct 30 16:09 mariadb_sys_schema.sql
-rw-r--r-- 1 root root 3327 oct 30 15:38 mariadb_system_tables_data.sql
-rw-r--r-- 1 root root 30685 oct 30 15:38 mariadb_system_tables.sql
-rw-r--r-- 1 root root 10506 oct 30 15:38 mariadb_test_data_timezone.sql
-rw-r--r-- 1 root root 1786 oct 30 15:38 mariadb_test_db.sql
-rwxr-xr-x 1 root root 9594 oct 30 16:09 mini-benchmark
drwxr-xr-x 2 root root 4096 nov 20 23:42 norwegian
drwxr-xr-x 2 root root 4096 nov 20 23:42 norwegian-ny
drwxr-xr-x 2 root root 4096 nov 20 23:42 polish
drwxr-xr-x 2 root root 4096 nov 20 23:42 portuguese
drwxr-xr-x 2 root root 4096 nov 20 23:42 romanian
drwxr-xr-x 2 root root 4096 nov 20 23:42 russian
drwxr-xr-x 2 root root 4096 nov 20 23:42 serbian
drwxr-xr-x 2 root root 4096 nov 20 23:42 slovak
drwxr-xr-x 2 root root 4096 nov 20 23:42 spanish
drwxr-xr-x 2 root root 4096 nov 20 23:42 swahili
drwxr-xr-x 2 root root 4096 nov 20 23:42 swedish
drwxr-xr-x 2 root root 4096 nov 20 23:42 ukrainian
-rw-r--r-- 1 root root 3474 oct 30 16:09 wsrep.cnf
-rwxr-xr-x 1 root root 4979 oct 30 16:09 wsrep_notify
If you have this dir with the language contents then:
Option 1
Create a new symbolic link pointing mysql to mariadb:
ln -s /usr/share/mariadb /usr/share/mysql
Option 2
Edit mariadb conf file /etc/mysql/my.cnf and replace this:
lc-messages-dir=/usr/share/mysql
by this:
lc-messages-dir=/usr/share/mariadb
Once done, restart mariadb.
systemctl restart mariadb