Well, the reason why I creaete this blog is to mark all the questions reagrding to user data, such as language, email, timezone, etc, to be synchronized in all places. I will constantly update the content of this blog to include all related topics.
Today, I will talk about language in user settings and su01 data.
In su01, there is two languages, one language under 'Address' tab and one logon language under 'Defaults' tab.
However in SRM User Settings, we only have one language under 'User Account' tab.
This is hard-coded in the method /SAPSRM/IF_PDO_MO_USER_ACCOUNT~GET_USER_ACCOUNT_DETAILS.
Here it takes ls_defaults-langu instead of ls_address-langu.
For language in default tab, it is mainly used for user logon. The language in address tab might be used in business processing such as email notification.
Of course, if you have your own business requirement, you may consider using enhancement to change this behavior, which is also quite easy.
To Be Continued...