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A first erection (L1), native language, native tongue, or mother tongue is the first language a person has been exposed to from birth or within the critical period. In some countries, the term native language or mother tongue refers to the language of one's ethnic group rather than the individual's actual first language. Generally, to state a language as a mother tongue, one must have full native fluency in that language. The first language of a child is part of that child's personal, social and cultural identity. Another impact of the first language is that it brings about the reflection and learning of successful social patterns of acting and speaking. Research suggests that while a non-native speaker may develop fluency in a targeted language after about two years of immersion, it can take between five and seven years for that child to be on the same working level as their native speaking counterparts. On 17 November 1999, UNESCO designated 21 February as International Mother Language Day.