PL/DEV SETTING:
DECIMAL SYMBOL = .
DIGIT GROUP SYMBOL = ,

NLS PARAMETERS:
nls_language = CROATIAN
nls_territory = CROATIA
nls_numeric_characters = empty (derived from nls_territory ,. )

NLS DATABASE PARAMETERS:
NLS_LANGUAGE = AMERICAN
NLS_TERRITORY = AMERICA
NLS_NUMERIC_CHARACTERS = .,

WHY PLDEV IN SLQ/WINDOW USES "derived from nls_territory" when I explicitly want ., ???????????

This is bad! I've allready had few "situations" with this.

EXAMPLE:
When I duplicate row in SQL/WINDOW results with COPY/PASTE, numbers are displayed as "123.45" but after submit requery shows that in database was stored "12345" !!!

If I understand correctly your decimal character is a "." in PL/SQL Developer (set through the preferences), but the Oracle Session is using a "," (derived from nls_territory). Is that correct?

Yes, but only when submitting data (WRONG) , not when fetching(OK).

Can you let me know your setting of the "Number fields to_char" preference (Tools > Preferences > SQL Window)?

Default (Green) (Not checked)

Any news Marco?

We have not yet been able to reproduce this issue.