phpmyadmin error "Unknown language: en-iso-8859-1

You need to check if the account having custom theme like RVskin? if yes then change the theme to default cpanel theme and the issue should be resolved.

For further support you need to contact RVskin support as cPanel wont support third party software issues.

phpmyadmin showing blank page

If phpmyadmin shows blank page then you need to run below script in order to get this issue resolved.

/scripts/makecpphp --force
 /scripts/upcp --force

How to optimize a MySQL database using phpMyAdmin?

It is always a good idea to keep your databases' tables optimized.

Fortunately, making this optimization is quite easy when using the phpMyAdmin tool available in cPanel.

To perform the optimization, log in to your phpMyAdmin and select the database whose tables you wish to optimize.

A list with all the database's tables will appear. Tick the tables you wish to optimize, or simply click [Check All] to select all tables.

From the [With selected:] drop-down menu choose Optimize table. This will execute the OPTIMIZE TABLE SQL query on the selected tables and they will be updated and optimized.