The case for "I have a doubt"
"Teacher, I have a doubt."
It is quite common for me to hear the above sentence spoken by my students. Just as common is to see posts and videos on Instagram and Youtube from foreign and Brazilian English teachers alike telling students, "don't say 'I have a doubt'". This advice is given under the pretense that it is wrong, or considered "bad English", to say "I have a doubt", and instead English speakers should say "I have a question." While I agree that the former sentence is probably more commonly used by native English speakers, I'd like to present the case for saying "I have a doubt".
Native speakers of a language know when something is correct or incorrect, not because they understand the reasons why, but rather because the correct answer simply "sounds right". Similarly, Brazilian students naturally say "I have a doubt" because it's a direct translation of what they say in Portuguese, and after teaching English in Brazil for a number of years, it has begun to sound normal to my ears. So much so that I often ask my students, "do you have any doubts or questions?". And while I understand the basis for telling students "don't say 'I have a doubt'", I think ultimately it is unnecessary and holds no ground.
Language is always changing, words are constantly taking on new meanings.
Languages are not static, rather they are alive and never cease changing. The only languages that do not change are dead languages. Speakers are always adapting their language, creating new words, giving new meanings to old words, etc. With time, many words take on new meanings. Using "I have a doubt" in lieu of "I have a question" would just be another such example of a word taking on a new meaning. No Brazilian English speaker would have any trouble in understanding what someone means when they say "I have a doubt", and I would even argue that other non-native English speakers would also have no such difficulty. If "I have a doubt" is already used and understood by so many English speakers, can it really be considered wrong? Certainly not here in Brazil! But maybe it could be considered wrong, or incorrect, according to other "standards" of English or regional varieties.
Standards are relative, what is considered (in)correct in one standard of English may not be considered so in another.
For something in English to be considered "wrong" there needs to be some kind of standard that dictates what is "right". There are countless different standards that apply in different contexts. For example, US and UK English follow different spelling standards; writing conventions vary greatly depending on what is being written. You probably employ many different language standards in your own life. Is the language you use with your friends and family the same language you use with your employer? What about the writing you use on Whatsapp versus in an email? While standards and conventions are important - they allow large groups of people to be able to communicate with each other clearly - they are also completely relative. "I have a doubt" may indeed be considered strange or even incorrect by some standards of English but I argue that, due to its widespread use and comprehension in Brazil, it should be considered perfectly acceptable according to Brazilian English standards.
There are many different varieties of English that reflect the culture and environment in which they are spoken.
"Brazilian English", it may seem strange to give a name to a non-native variety of English, but why not? There are many varieties of English spoken worldwide that reflect the culture and environment in which they are spoken. Many of these varieties are spoken as a second language and by far greater numbers of speakers, are they any less authentic than native varieties? Language is always changing to reflect its speakers, and speakers will be reflected in their language regardless of whether it is their mother tongue or not. In the present case, I am not arguing in favor of the adoption of grammatical structures that would be considered grossly incorrect by other native or non-native varieties of English, nor am I suggesting that Brazilian English should not follow generally agreed upon English standards. I am simply arguing in favor of an added use of the word doubt which reflects Brazilian English standards and the idea that considering "I have a doubt" to be incorrect is unproductive at best and harmful at worst. Such statements are generally disseminated by native English speakers who use their native status as an English speaker as a badge of authority over a language which does not belong to anyone.
Why are native English speakers considered better than non-native English speakers?
Native English speakers are often viewed as authorities when it comes to English, but is there any reason to consider them as such? English is an international language, it is spoken by more non-native speakers than native speakers. As an international language, the goal of non-native English speakers should not be to speak just like natives but rather to be understood by the international community. Furthermore, given that there are so many native varieties of English, how could any one native be considered an authority on the subject of English? At most they can only be authorities on their regional variety of English. So while some native English speakers may consider "I have a doubt" strange, there is no need for non-native speakers to adapt to these standards, especially considering that they would be understood by most, if not all, international English speakers.