When using SQL Server, your current connection includes a number of options that determine things like the language, date formats, etc. These could be set at whatever the default is, but they can also be overridden during the session by using a SET statement.
The date format affects the interpretation of character strings as they are converted to date values for storage in the database. When the language is set using SET LANGUAGE, the date format setting is implicitly set accordingly. This can be explicitly overridden with the SET DATEFORMAT statement.
In any case, you can find the current date format by using the DBCC USEROPTIONS command. This command returns the SET options that have been set for the current connection.