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One who does not understand the words in the Megillah: 
One who hears the reading of the Megillah in Lashon Hakodesh fulfills his obligation even if he does not understand the words.
 This applies even initially. [M”B 690/26; Kaf Hachaim 690/47]
The reason: As even those that understand Hebrew do not know the meaning of every word, such as the word Achashdarnim. The reason why it is not required that one understand the words being read is because the entire Mitzvah is the reading and publicizing of the miracle. It is for this reason that women and children also fulfill their obligation. [Taz 690/6; M”B 690/26; Kaf Hachaim 690/46]