the language with which the speaker has greatest proficiency and/or uses most often
Any language dominant in the child's situation, often (but not always) it is the community language.
A language minority student's most developed language.
This term has two distinct meanings: (1) the language of primary competence, the language that a person knows best, which may or may not be the person's native language. For example, in the case of immigrants educated primarily (or exclusively) in the US, although their native language may be other than English, their primary language competence is in English, not in their native language.(2) the language spoken by the dominant class, the recognized standard of correct grammar and usage. Some examples would be "the King's English," "New York Times English", the Spanish prescribed by the Real Academia, etc. The more usual term for this is standard language.
Language of habitual use