According to this document:
the default value is AMERICAN_AMERICA.US7ASCII
Here is another helpful document about NLS_LANG:
Correct. On non-EBCDIC platforms (which are almost all platforms) the default is AMERICAN_AMERICA.US7ASCII.
US7ASCII has specific replacement characters defined for certain accented letters. Therefore, some accented letters converted to US7ASCII will become '?' and some will become the corresponding non-accented (base) letter.