Ordering your coffee in French, answering an email in English, and speaking another language at home – this is the reality for many people in Brussels.
As one of the world’s most diverse and multilingual cities, the Belgian capital is home to people who often speak two or three, and sometimes even four languages. For many residents, switching between French, Dutch, English, and another language is part of everyday life.
According to data from BRIO Brussel, the Brussels-Capital Region was home to more than 180 nationalities in 2024, with residents speaking 104 different languages. The most widely spoken languages are French (81%), English (46.9%), Dutch (22.3%), Spanish (14.5%), Arabic (11.5%), Italian (6.1%), German (6.1%), Portuguese (2.8%), Turkish (2.8%), and Romanian (2.1%).
Speaking more than one language has obvious advantages, allowing people to communicate across different communities. Multilinguals may make it look easy, but in reality, things are not so simple. Scientists say those benefits also come with hidden cognitive costs.
What happens in the brain?
When multilingual people speak, they often activate words from other languages they know, explained Professor Mathieu Declerck, senior researcher at the Vrije Universiteit Brussels (VUB). His research focuses on multilingual language processing.
"Dutch and English are the languages I know best, so whenever I want to say something in English, I'm also activating words in Dutch," he told The Brussels Times. "For example, when I want to say the word 'dog', the Dutch word 'hond' is also activated in my brain and they are both competing to be selected."
According to Declerck, this constant competition has to be resolved in some way, which he says is done through language control. The mechanism allows people to select, switch between, or suppress languages as needed. This process is explained through the concept of inhibition – suppressing the language that isn’t relevant at a given moment.
When switching, “your brain assumes that there’s a higher possibility of making an error because of the competition between languages,” says Declerck. He believes this happens because our brains are more alert to potential mistakes.
Cognitive effort
Language inhibition requires considerable cognitive resources. Suppressing one language leaves fewer mental resources for producing the one that we are trying to speak.
This often happens when people move between conversations with family members, with whom they speak one language, and interactions with people who speak another.
Another theory, according to Declerck, focuses on overcoming the inhibition of a language. “It takes a lot of time, and it causes a lot of errors,” he says.
While these two theories differ, they are not, as the scientist pointed out, “mutually exclusive”. These notions can exist simultaneously but they both support the idea that the language that is no longer needed has to be suppressed to some degree.
Another characteristic of life in Brussels is having to speak a language we are not fully fluent in. If you have to speak French, Dutch or English and you don’t feel 100% comfortable, then your brain will often translate from the language you feel most confident speaking.
“This is a huge mental effort,” says Declerck, and it also one of the reasons why we get so tired after speaking that language. “It’s obviously not the most economical way but that’s how the brain works."
The cost we pay
Shifting between different languages comes at a cost that “appears in many different shapes and forms,” according to Declerck.
One of the most common consequences is making semantic errors. For example, if you want to say “horse,” you accidentally say “cow,” he explains. “The most common error from switching between languages is actually using the wrong language."
Another concept Declerck explains is "language intrusion", which is the involuntary mixing of words and grammar from one language to another. It can be as subtle as accidentally replying in the wrong language or struggling to form a sentence.
Other issues include pauses, stutters or failing to switch languages altogether. “Sometimes people don’t transition to another language, even when you ask them to and they stay in the language they are currently speaking,” he says.
However, Declerck highlighted that when we switch voluntarily, the cognitive costs are much smaller. “If you’re speaking your native language to somebody, you might switch sometimes to English because it feels more appropriate,” he adds, arguing that one of the biggest misconceptions is the belief that language switching happens automatically. “You don’t have to make a huge conscious effort, but people assume this is super easy and requires no cognitive effort at all."
Moreover, it is more common for people to be more emotional in their mother tongue than a language they have learned in a more “clinical environment,” says Declerck. This is because you tend to learn your native language in an emotional environment – at home with your parents – while other languages you learn through TV or school.
