My wife and I are both born and raised with just one language - my wife is from Austria and I am Dutch. We met in London, got married in Ealing and we have 3 year-old boy/girl twins.
When our twins were born, we made the conscious decision that the twins should learn both our native languages as well as the 'local language' (English). We felt, that apart from being part of our culture, the importance of our native languages is that our siblings have young children too and we wanted to ensure that the twins would be able to communicate with their cousins and grandparents. We live in the UK so there was never any doubt that the local language would also become an important part of their language repertoire. We speak English to each other and everything outside the front door is in English :)
Although inexperienced, we were convinced our approach was right for us but, as time went by, there have been instances where others started to question if we were doing the right thing for our kids. Around Christmas last year, some people started suggesting that maybe we should consider dropping one of the languages as the twins were over 2 years old and only starting to mumble their first words. They were all really focused on the vocal part of their own language and didn't realise that the children understood every instruction given to them, irrespective of the language it was delivered in, Dutch, English or German. This was also when we came across a funny 'problem'. When we looked at pictures with the children and they would name something, e.g. a tree, but they call it a 'Baum' (German) when they talk to me, then technically it's incorrect, as they should use the Dutch word instead. However, it's not really incorrect, because they've learned the German word and that should be emphasised as being correct - even by the 'wrong' parent. We found it very useful to say: 'Yes, that's what mummy calls it. Daddy calls it... and your friends call it...'.
Since Christmas last year, so many things have happened. The twin have made up so much ground and are currently speaking multiple word sentences using a wide variety of words in all three languages. They confidently switch languages between talking to mummy and daddy and are perfectly happy to 'translate'. If I ask them "Can you please ask mummy ..... " (in Dutch), they run off towards mummy shouting ... uuhhh ... repeating my instruction in German.
They are in pre-school now where their English vocabulary is booming. It is very funny to see how they are now interacting as they use all three languages. Which language is largely determined by what they are doing at the time. Some games they do mostly with mummy and will be done in German, some puzzles are done with Daddy and will be 'discussed' in Dutch whereas some toys are associated with pre-school so they talk English.
I would love to say we knew exactly what we were doing these last few years. We believed in what we were doing and stuck to it. Some of our biggest challenges were to overcome other people's misconceptions and to figure out which language the children were trying to use. Now, their speech is becoming better, we can distinguish the language easier (very helpful), but ... we are still fluently ignored in all 3 languages.