It seems that the term 'first language', 'native language', 'mother tongue' each may have some different connotations or unnecessary implications of political correctness to some people. 'L1' seems to be a strong candidate, but I'm afraid some non-English native speakers may not understand the term instantly. I'm running some survey and want to add this question to a group of people most of whose L1 is not English.
Key the following into Wikipedia search and you would land yourself at the page http://en.wikipedia.org/wiki/First_language.
- Primary language
- First Language
Otherwise, you might also use
- Most-proficient language
- Most proficiently written or spoken language
Therefore ask the person ... What is your - Primary language - First Language - Most-proficient language - Most proficiently written or spoken language
Having or marked by an advanced degree of competence, as in an art, vocation, profession, or branch of learning.
A person who exhibits such competence; an expert. [Latin prōficiēns, prōficient-, present participle of prōficere, to make progress; see profit.] pro·fi′cient·ly adv.
proficient, adept, skilled, skillful, accomplished, expert These adjectives mean having or showing knowledge, ability, or skill, as in a profession or field of study. Proficient implies an advanced degree of competence acquired through training: is proficient in Greek and Latin. Adept suggests a natural aptitude improved by practice: became adept at cutting the fabric without using a pattern. Skilled implies sound, thorough competence and often mastery, as in an art, craft, or trade: a skilled gymnast who won an Olympic medal. Skillful adds to skilled the idea of natural dexterity in performance or achievement: is skillful in the use of the hand loom. Accomplished bears with it a sense of refinement after much training and practice: an accomplished violinist who played the sonata flawlessly. Expert applies to one with consummate skill and command: an expert negotiator who struck a deal between disputing factions.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.