Your kid is now introduced to 2-3 languages in school apart from their mother tongue. Your kid is slowly developing multi-lingual learning experience, but first and foremost, his mother tongue is going to have a strong influence over whatever new language that he learns.
What you need to know
Learning to read and write in the mother tongue is more beneficial for children to learn other languages easily as shown by certain studies. It is also the basic language they have learnt first and thus can express their feelings better and have a closer relationship with their parents, siblings and friends in their mother tongue. As a parent do not downgrade speaking in the mother tongue in public to children this age as it deprives them of expressing their feelings and lowers their ability to converse.
Some schools have the local language as part of the curriculum but some do not. In those cases, encourage your kid to learn the mother tongue at home.
What you can do
The mother tongue is a part of the child’s personal, social, and cultural identity. A kid this age has already been taught to express their feelings, converse with you in your tongue. For the sake of society and other reasons do not curb them from speaking in that language. Fluency in the mother tongue provides a linguistic foundation to learn other languages. Being multi lingual helps the children to gain the ability to process the information they receive in more than one language which enhances their flexibility in thinking.