import re
text = "Text1.\n Textt « text2 »\n Some other text"
print( re.sub(r'(\.)[^.]*«[^«»]*»', r'\1', text) )
aW1wb3J0IHJlCnRleHQgPSAiVGV4dDEuXG4gICAgICAgIFRleHR0IMKrIHRleHQyIMK7XG4gICAgICAgIFNvbWUgb3RoZXIgdGV4dCIKcHJpbnQoIHJlLnN1YihyJyhcLilbXi5dKsKrW17Cq8K7XSrCuycsIHInXDEnLCB0ZXh0KSAp