This issue will be resolved in a future version of Oracle APM.
The NLS_LANG is only used by the plug-in to query the list of tablespaces and other items from a database.
It is used exclusively for queries and only for display purposes. The actual data that is backed up trough RMAN is not affected by this setting.
The backup data is actually comprised of blocks of data that RMAN will send to NetVault media and the NLS_LANG setting does not have any role or impact on this block transfer.
The NLS_LANG would only affect how the names of the tablespaces are shown in the User Interface.
Because of Bug 28337, users may not see the correct NLS_LANG option setting, however, the chosen option is correctly stored.
To verify this, edit the following config file:
Look for the '[Oracle:Select NLS_LANG]' stanza.
Under this stanza check that the 'Value=' field has the desired value.
Alternatively, you can simply edit the database settings again and configure the required NLS_LANG setting.
However, be aware that every time the edit database GUI dialog is brought up, the wrong value will be shown but this can be safely ignored if the configuration file shows the correct value.