I have a little bit of experience in this. Brazilian living in Quebec. So, my friends almost always spoke in Portuguese with their children, school and TV were in French. After some time, school starts also to teach English and they chose the language of the TV. Now the kids are almost always speaking in English, although the are fluent in Portuguese and, of course, French.

Now, my wife and I hated the TV in French, so we kept it in English. So, my kids had to deal with three languages from the start. They mixed everything up and we screwed up by saying words in all three languages. We speak mainly Portuguese but we would use words that they learned in other languages in those languages instead of in Portuguese. In the end, my kids mix all three languages in a single sentence, which is weird as hell. They’re slowly separating the languages and we too. Now, every sentence we speak is in a single language. Their friends help in a way, because they also speak French in class but English outside (and Quebec’s government hates that).

So, if both of you are Portuguese speakers, I’d only speak Portuguese with them and let the TV and school teach English. They’ll know how to keep things separated.

Not a parent, but I was raised bilingually in English and German, while growing up in Germany.

My Dad (almost) always speaks English with me, and my Mom (almost) always speaks German with me, even to this day at age 31. This approach worked well for us and I'm fluent in both languages, but I can imagine an approach where both parents speak both languages could work as well.

What also really helped me was to consume a lot of media in English, so maybe you could encourage your child to do that as well.

My parents don't speak English, but I learned it as a kid by watching a lot of Cartoon Network. All the cartoons were in English, no subtitles or dub or anything. Somehow I assimilated the language without any external aid, and then learned the rest when we first got the internet and I started communicating with others via games.

So, if I had to teach a kid English, I'd just expose them to as much English as possible with plenty of context and encourage them to express themselves in English when they can. This is also a popular method how adults can learn languages, called tprs

We speak Irish to the kids as much as possible, essentially all the time. Them learning English is a given, a force like gravity.

We try to get them to read Irish books, watch Irish cartoons, but that can be a struggle with the temptation of English-language ones. Children have their own strong preferences about those things.

You need to take advantage of code switching, similar to how you'll use curse words in some contexts, but not in others. Or retail language vs casual.

They should have some intuitive idea of when to speak Portuguese and when to speak English. If you're mixing within the same context, that will be difficult.

Former child in a bilingual household. The time that your child spends outside of your home has by far the biggest influence on language fluency. You can have your child speak a language at home, and they would be able to understand it and speak it, but it would be limited - likely conversationally fluent, but not natively fluent.

If you can find a community for that language and culture that you visit every once a week, it will help reinforce that language. There might be language schools run by people from that culture - it'll be an easy way to get in touch with other people from that same culture

I was born and raised in the US. My parents spoke Spanish only. They maybe knew 5 words in English. I grew up learning Spanish fluently at home. Everything else (i.e. school, tv, friends) was in English, so I learned that naturally.

Downside: I feel like it created 2 personalities. I feel emotions, relate to family and romance, and cook in Spanish. I think logically, conduct business, and have friends in English.

Here in Hong Kong, we have three official languages: English, Cantonese, and Mandarin (okay, two only if you count Canto and Mandarin as one language, in which case dllm), and all three are taught in schools (Mandarin generally up to grade 9, the rest up to grade 12).

Almost all of us grow up trilingual.

I have a two year old. I speak German, partner speaks French, we speak English to each other (but not the kid), and we live in Sweden, so the kid’s learning Swedish at the daycare.

So it’s 4 languages (3 that we teach plus English) and the one parent one language approach.

Kid was a bit slow to start speaking, but now he understands a lot in all 3 languages, learns new words in all 3 all the time, and even picks up a bit of English occasionally. He’s started to distinguish the languages too, depending on who he talks to, but it’s definitely usually still a mix of all languages. When he speaks Swedish to me, I either just reply in German, or I might repeat what he said but in German first. And when he asks for his favorite lullaby in French, I just tell him in can’t do that one. We also have books in the different languages, but we might just use them in either language and describe what’s happening instead of reading it out.

And I’m told this mixing improves over time, I’m not worried at all. So I would say this approach works really well for us.

If you mix the languages between both parents, I think (but that’s just my gut feeling), the kid will have an easier time distinguishing the languages later on if you associate some activities with a specific language. Could be a place where you always use one language. Or some books, etc.
But, I doubt that’s a must. Kids are astonishingly good at learning languages (I’m so jealous).

So we have 3 languages in the family, native languages for me and wife + English as common language. It's a huge mess and at almost 4yo she's mixing up languages and a little bit slow on learning to speak in general, but we've been told that's just how it goes

My friend is French, his wife Portuguese, they live in England with their two children. When all together, they all speak English with each other. When the kids are with one parent, the speak that language. In the park with father, French. Baking with mother, Portuguese. Bedtime stories are in the language of the parent reading. Kids switch between languages easily and understand what to speak with whom. Effortless trilingual.

Another friend moved country with her husband and had three kids. Home language was always mother tongue, both my friends had fairly bad English. Everything outside parents is in English for the kids - media, school, anyone outside the household. Again, the switch for the kids is really easy, they are fluent and have no accent in both languages.

I'm bilingual.

They just sent me to an English daycare and spoke French with me at home.

Then I could do the rest on my own.

I grew up in a multigenerational home. Grandparents spoke one language, my parents spoke another. Used to play with the neighbours a lot and picked up a third language from them. Then started elementary school and learned a fourth there. It seems to work well to have each person in your life exclusively using one language.

How well this goes depends on a lot of factors: are those languages native to either parent? What language is spoken where you live? Do you have other people in their lives that speak these languages? Are there other contexts in which those languages are spoken beyond the home (social occasions, TV, etc)?

Apparently, for it to really stick, it takes a lot more than just a parent speaking. I recommend listening to this podcast episode with a researcher that runs a bilingual child development lab. TBH, it's a bit disheartening to hear how hard it is to make it work: https://yourparentingmojo.com/captivate-podcast/bilingual/

The questions I asked above come from listening to that. Another big takeaway is consistency. One parent should stick to only one language talking to the kid.

I live in the US and I am a native Portuguese speaker, and my wife is a native Farsi speaker. We both spoke our own languages to our kid, and at age 2 he would mostly only speak those languages, and would even translate between them naturally (like I would say "go tell mom X" in Portuguese, and he would go and tell her in Farsi). But at age 3 he started just replying in English... Even went to Iran at 4, and could understand all his cousins but only replied in English. Farsi had a better shot because he has more exposure to it than Portuguese, but still... Honestly, it's one of my bigger disappointments in my parenting because it was really important to me, but I myself fumbled with it: when he started speaking in English to me, I started sometimes mixing it up and responding in English, which is not good for this (I have lived in the US since high school, so it's honestly a little easier for me at this point too). I was also a little concerned about his development in English and communication with his friends in school, but that's not necessary, that will come no matter what, so stick with it. My brother also lives in the US and is married to another Portuguese speaker, so his 2 kids born here speak it just fine since it was the only language at home. Their grammar and vocabulary is a little weird, but they can get by just fine.

Edit: sorry for any repetition, when I went to comment I couldn't see any other comments for some reason and thought I was the first to respond

Kids are like sponges, especially when it comes to languages.

We lived in Romania until a few years ago, when we moved to Germany. We speak Romanian at home, and they have a decent grasp. The oldest is fluent, with no accent, the youngest lacks vocabulary, applies German grammar rules and has a bit of German accent. All of them speak German fluently, and they use it when they talk to each other, with friends and at school. They are also fluent in English, and that is because of the media. I only use English when it comes to devices OS/interface, streaming services etc, and they are used to it. They consume YouTube, Netflix etc in English only.

They sometimes mix the languages, especially when talking to us, since their exposure to Romanian is limited.

I was raised bilingual, and speaking from my own experiences I'd say that it's a good idea to consider the following questions if you want to maximize the child's ability in either language:

1. Is there a parent who the child sees more or less often than the other? What will one do in case one parent dies, or in case the child has a language disorder, or there is otherwise some sort of unexpected problem that could impact the child's language development?
2. What are the language dynamics at play in the family and in the local area? What will the child associate with each of the two languages? Can the child have all its needs met in the non-dominant language? Does the child have access to a broader community of speakers, and in what way?

I'm not sure if it's a good idea to share my own story because it gets pretty melodramatic at points, but yeah, language skills need to be built and maintained over the course of one's entire life, so you need to be able to adapt to changing circumstances. But as a whole I think that what you're planning for your own kid sounds like it will work well, or at least decently well — the only way to know for sure is to get a time machine and go forwards 20 years, and until then I think it's best to have faith in your competence as a parent. There's no-one who knows a family better than itself.

And beyond that, one should also ask oneself... Well, what types of language skills does one want to see in one's child, and what happens if the child ultimately does not reach the goals one has set? I'd say that I have sort of a nuanced or over-complicated relationship to so-called "bad grammar" because of my position.

French and English in school, Mandarin course on Saturday. Cantonese with parents,

At least that was my upbringing

I moved to the Netherlands when my boy was 8yo. At that time he was fluent and capable to read and write in Portuguese. Now, two years later he can also do the same in Dutch, but the challenge now is that he is forgetting some words or some meanings in his mother tongue. I ask to everyone coming from Brazil to bring books for him, which helps a lot (he loves to read). Besides of that, it's super hard for me to follow up him with his homework.

When I was the kid, the house rule was French at home, English everywhere else. Kind of like your plan but stricter. Now that I’m the parent, the kids are in French Immersion school, so at home we do 50/50.

the hardest part was finding a doctor willing to cut my newborn children's tongues in half