English is my mother tongue, my first language: it's the language we have always used at home—both the home I was raised in and the home I raised my children in—and it's the only language I know well. (This is not something I'm proud of—I wish I'd paid more attention to languages like Bengali and Malayalam.) This situation came about because my mother, who was raised in Malaya, started learning Malayalam (no, Malayalam is not the national language of Malaya!) only after her marriage, and so my parents began to use English at home (this scenario occurs often in inter-community marriages—which my parents' marriage was not—with English serving as the link language in the family). In official forms I give my mother tongue as Malayalam, because in official circles the mother tongue is often used as a proxy for the state of origin, and sometimes for the community/ethnicity (as far as I know, in India, only Anglo-Indians have English as their official mother tongue).  Some bureaucrat may draw the wrong conclusions if I give English as my mother tongue. 

English is an international language, the most cosmopolitan of languages, and it began to drift inexorably from its Anglo-Saxon moorings when the British dispersed all over the globe. It is a rich language "that has been made and remade over centuries in different countries by different cultures" (Alexandra Pringle in Literary Review, The Hindu of October 17, 2021). English is today an Indian language as much as it is a British and American language, and there are many like me in India who think and dream in English (though their official mother tongue is some other language). India is a multilingual country, and most Indians are proficient in more than one language; three languages are taught in most schools. (My father once observed that at the age of 3 or 4, I spoke in Bengali with my ayah, switched to Hindi with the cook, and reverted to English with my parents.) The medium of instruction in many schools is English. The roll call of distinguished Indian writers in English is long, from Jawaharlal Nehru, Mulk Raj Anand, R. K. Narayan, Ved Mehta, and the naturalist M. Krishnan to Salman Rushdie, Arundhati Roy, and Amitav Ghosh.

The idea of a "native speaker" of a language is simple: the native speaker is born into a particular language and grows up with it. This concept is crystal clear in predominantly monolingual countries; in predominantly multilingual countries like India, it can become fuzzy: multiple lingual nativities can happily coexist. As the American translator of Hindi literature, Daisy Rockwell says: "There is a fluidity to the South Asian language-scape that is wholly lacking in the United States, which is, despite the diverse population, ferociously monolingual. Code-switching, the practice of sliding effortlessly from one language to the next, or mixed idioms, like Hinglish, are practically non-existent in the US, outside of immigrant communities" (in Gupta T., "Meet the American who translates some of India’s finest Hindi writers into English," Scroll, May 13, 2018). 

I copyedit mostly in US English and sometimes in UK English. Experienced copyeditors can usually work with more than one variety of English. I hardly ever work in Indian English, because my end clients are Western publishers/research organizations. Scholarly publishing is a Western-dominated enterprise. My domain is academic writing, which employs (or should employ) a form of standard English that is universally understood.