We've got a Samsung Galaxy S from another region (China or Korea).
What needs to be set up in it for a Russian user in Russia?
The obvious group of settings that needs to be changed (simply by selecting the appropriate value) is the interface language, time zone.
One unexpected difficulty is the input method.
The factory Samsung OS (from another region) doesn't include a Samsung Russian keyboard, nor does it include the standard Android keyboard (which might have a Russian keyboard). Further complication: The standard Android keyboard cannot be installed separately!
As a solution, I chose to install AnySoftKeyboard (which is also available on F-Droid, which guarantees it is free/open-source, can be rebuilt from sources, and has been inspected by more people for not having spying features). Then in its settings menu, I chose to download a Russian pack.
Mobile networks and internet
For internet to work, one needs to set up an APN (for example, for Megafon, a Russian mobile operator, one needs to add a new APN with "internet" as the value for the field "APN" -- the only essential field; and select it).
(Still, as compared to Samsung Galaxy S sold in Russia, this one has much more unstable connection to the mobile network (loses the connection, i.e., is offline very often). Perhaps, this can be solved by upgrading the ROM/modem firmware from the present Android 2.2 I9000XXJPK (kernel 188.8.131.52) FROYO.XXJPK.)