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Hi,
>>>Is it possible to alter the right click menu in AutoCAD 2015 for MAC?
Officially it is not possible. You need to edit CUI files (menu files) manually.

>>>is it possible to change the language of my
>>>english version of AutoCAD 2015?

There are only two language versions of AutoCAD for Mac - English and French, all other is not available at all.

Maxim