Language development, language learning
First and later language aquisition
L1 refers to your native or first language (sometimes mother language, mother tongue)
L2 refers to your second language (sometimes auxiliary language)
Whether we use L2 for 'all others' or whether we continue counting depends a little on context.
Note also that this does not necessarily correspond to the dominant language, the one the speaker is most comfortable with.
Methods
A lot of learn-a-language books focus on getting you to produce a few common sentences and/or rote learning, without necessarily really understanding the underlying decisions.
See also
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