Sp_Login: Transferring Users/SIDs Between Servers

My question is, why did I have to use the allegedly optional named argument @DefLanguage when using sp_AddLogin? I had often tried to use the @SID parameter of sp_AddLogin to synchronize users betwixt our servers. Previous efforts had failed.

Per Transact-SQL Help, here’s the syntax for AddLogin:

sp_addlogin [@loginame =] 'login'
[,[@passwd =] 'password']
[,[@defdb =] 'database']
[,[@deflanguage =] 'language']
[,[@sid =] 'sid']
[,[@encryptopt =] 'encryption_option']

We have over a dozen User DBs, and I’d written a cursor to add the login and then GRANT access to the appropriate DBs. I used the following variables:

DECLARE @Loginame VarChar(25) , @Passwd VarChar(25) , @DefDB VarChar(50)
DECLARE @SID VarBinary(16)

Then I SET the variables:
SET @Loginame = 'MyUserName'
SET @Passwd = 'UsersPwd'
SET @DefDB = 'UsersDB'
SET @SID = 0x6370170BDAAFF640AD5CEB586EA87C2C
-- UserSID for MyUserName from sister server

Then I EXECuted the proc.
EXEC sp_AddLogin @Loginame , @Passwd , @DefDB , @SID

This generated the following error (instead of ‘?’, I saw squares, as in unprintable characters):

Server: Msg 15033, Level 16, State 1, Line 0
'????????' is not a valid official language name.

I didn’t understand this at all at first, but out of desparation, I DECLAREd & SET the @DefLanguage argument...And it worked! That is:

DECLARE @Loginame VarChar(25) , @Passwd VarChar(25) , @DefDB VarChar(50)
DECLARE @SID VarBinary(16)
DECLARE @DefLanguage SysName

Then added:
SET @DefLanguage = NULL
-- per sp_Configure, RunValue = 0
-- Master..SysLanguages reveals that language ID = 0 corresponds to ‘us_english’

Then I EXECuted:
EXEC sp_AddLogin @Loginame , @Passwd , @DefDB , @DefLanguage , @SID

Success!

Again, my question is, why did I have to use the allegedly optional named argument @DefLanguage?