Check whether your database, applications and operating system are using compatible charactersets. Predicting the results when when storing an 8 bit characterset in a 7-bit database is virtually impossible. The characterset defined in NLS_LANG parameter does not change the client's characterset, it lets Oracle know what you are using on the client side so Oracle can do the conversion. There are both 7-bit and 8-bit Arabic encoding schemes.
Check out the following notes in Oracle Metalink: Note: 70117.1 - Create A Translation Using Translation Builder (Only applicable for AR8MSWIN1256 Character Set)
Note: 158577.1 – NLS_Lang explained.
Note: 226692.1 – Finding out your NLS setup
Note: 241048.1 – The priority of NLS parameters explained
Note: 225912.1 – Changing the Database Character Set - an Overview
Note: 66320.1 – Changing the database characterset or the Database national character set
Note: 13854.1 - Dump SQL command for NLS debugging