I would like to know the ruling from the Quran and the Sunnah in regards to a group of people who constantly speak their native language in front of others who do not understand this language.
Can you please provide the proof for this?
Thank you

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If there is only one person in a group who does not understand a language that the others speak and if the group is able to speak the language that this person understands, then speaking the language that person does not understand can be considered as trying to hide something from him. This is of course forbidden. Imam Al-Bukhari and Muslim reported that Abdullah narrated that the Prophet (Sallallahu Alaihi wa Sallam) said: "if you are three people, then two of you should not speak confidentially unless you mix with people, so that this will not upset the third person" and in another narration" that (speaking secretly) harms the Mu'min and Allah does not like that the Mu'min be harmed".

However, if the people who do not understand the language being spoken are more than one, then there is no harm in that case to speak in this language, but it is more appropriate to speak the language that everybody understands, because this is more friendly and more respectful.

Allah knows best.