From http://www.weberp.org/ download the desired language pack zip files and unzip them in your WebERP instances' "locale" directory. Then change ownership of all files to www-data.
Make sure the server has a locale for each of the languages you want to use:
Note that there must exist a system locale that is EXACTLY THE SAME as the WebERP locale, ie. zh_CN is NOT the same as zh_CN.utf8.
Now you can login to WebERP as admin and create users with language defaults for any of the supported languages (which will be apparent in the language drop-down list in the user profile editor).
Note that I had to install the ttf-arphic-gkai00mp package to get simplified Chinese (zh_CN) working, but Firefox and Opera do not seem to autodetect the GB2132 font well and encoding must be selected manually from the browser menu.
Download the latest version from http://www.weberp.org/
Create a directory for webERP and copy the downloaded zip file into that directory, and unzip it. (Do not try to unzip from a parent directory, as the last time I tried, webERP unzipped all the files into the current directory, not a subdirectory.)
Correct the ownership of the files:
chown -R www-data:www-data ../webERP_3.10.3/
Have a look at the installation/upgrade notes in webERP_3.10.3/doc/. For new installations, we must create a database and user for the new installation first, manually:
mysql -p Enter password: mysql> create database apps_weberp; mysql> GRANT ALL on apps_weberp.* TO 'apps_weberp'@'localhost' IDENTIFIED BY 'appsPassword';
Then edit weberp-new.sql to add a line at the top:
where apps_weberp is the name of the database you just created, and then from the shell command line (not from the MySQL command line) run:
mysql --user=apps_weberp --password='appsPassword' < /var/www/vsc/apps/webERP_3.10.3/sql/mysql/weberp-new.sql
to import a clean new database (NOT the demo database). A "show tables;" on the new database should now show a lot of tables. Create and edit the config.php:
cp config.distrib.php config.php
Change the following settings in config.php:
$dbuser = 'apps_weberp';
$dbpassword = 'appsPassword';
Per  Q14, webERP supports having one or more companies using the same instance of webERP. Inside the "companies" directory, there is exactly one subdirectory per company, with the name of the subdirectory exactly the same as the corresponding MySQL database. (I am assuming then that there is only one MySQL user per instance of webERP, and that single user controls all the databases / companies associated with that instance.) The default install has a single subdirectory named "weberpdemo" in the "companies" directory, so this must be renamed to agree with a MySQL database:
mv weberpdemo apps_weberp
The subdirectory names under the "companies" directory are apparently what is used to populate the "company" drop-down menu on the login screen. If the company you select does not have a corresponding database, you will get the error:
"The company name entered does not correspond to a database on the database server specified in the config.php configuration file"
which is actually a bit non-sensical because it has nothing to do with the config.php file.
Now you should be able to login as the admin user with default password of "weberp". Change that password. And in config.php:
$allow_demo_mode = False;
to get rid of the password display on the login screen.