Is it OK to Change System Locale after MSSQL2012 installation
In general you should be ok. The only area that I am aware of that could be affected is SQLCLR, in which case the following conditions must be true:


(
The LCID of the OS does not match the LCID of the default Collation of the Database where the Assembly is loaded,   

OR  
The default Collation of the Database where the Assembly is loaded is either a binary Collation, or is any combination of the following: case-sensitive, accent-insensitive, width-sensitive, or Kana-type-sensitive
),

AND  
custom SQLCLR code is being used
SqlString is being used as an input parameter type
The SqlString parameter is being concatenated inline with a string while not using either the Value property or the ToString() method (i.e. "some string" + SqlStringInputParam;)
Or, possibly using Regular Expressions / Regex.


To be clear, this issue relates to the default Collation of the Database in which the Assembly is loaded, not the Instance- / Server- level Collation, nor even the Collation of any string column or any string literal or variable with a Collation set via the COLLATE keyword.

If your Database has a default Collation of SQL_Latin1_General_CP1_CI_AS because that is what the Instance-level Collation is, then neither # 1 nor # 2 above are true. And in that case, IF there is any SQLCLR code in that Database that matches items 4 - 6 above, then you wouldn't see any problem. BUT, changing the Locale of the OS would make item # 1 above true, in which case if there is any SQLCLR code that matches item 4 - 6 above, then that code will start to throw "Collation mismatch" exceptions.

Admittedly there is a fairly low probability of running into this. But, it has happened, so it's best to be aware of it.