He's an arrogant Westerner who insists his own dictionary is right, despite not even grasping Japanese nuance.
Literal
Japanese [gen-の] nuance [subject-が] don't-understand [despite-くせに], self [gen-の] dictionary [gen-の] side [subject-が] correct [quotative-と] insist arrogant [attr-な] damn-Westerner is.
Note the slur: 毛唐 (けとう) is a dated, derogatory term for Western foreigners, literally 'hairy T'ang person.' JMDict flags it as offensive; learners should recognize it but never use it. くせに attaches to plain-form verbs/adjectives and means 'despite X / although X,' always with a tone of contempt — stronger than neutral のに. 自分の辞書のほうが正しい uses the ~のほうが comparison pattern ('one side is more X [than the alternative]') without making the other side explicit.