SAP Language Pack Enabling for Login in R/3 Enterprise 4.7

In an SAP system you can see what language packs are installed using transaction SMLT.
Sometimes however, you may find that you are unable to log into the system using one of the languages that are installed.  You get prompted to select a valid language.
This is usually due to a miss-configuration of the zcsa/installed_languages profile parameter.

You can check the system language configuration using the report RSCPINST.
You should note that the zcsa/installed_languages parameter should be set in the DEFAULT profile *only*.

Some useful SAP notes on language packs:

73606 “Supported Languages and Code Pages”
352941 “Languages and Support Packages”
533888 “Example for Language Import and Errors”