In order to give the appropriate answer we need to know what is being asked exactly. If the question is asking about why some other language wasn't chosen to pray instead of Arabic, the same question will come up for those who don’t speak that other language. But if the question is about why we have to perform prayers only in Arabic and why it isn't permissible for one to pray in his/her mother language, then we need to pay attention to the following points to get an answer to the question:
1- The main reason behind performing prayers only in Arabic after the fact that it is part of the prophet’s (pbuh) sunnah and that in general, all forms of worship need to be dictated to us by Allah (swt) the way He wants them to be performed, is to avoid the alteration of the form of prayers throughout the different ages and to be protected from any potential change, and if every individual is to perform prayers in the language he/she speaks there would be a fairly great possibility of the alteration of the prayers by adding or omitting some words or even mixing it with superstition, there were also chances of these changes would leading to other changes in the fundamentals of the prayer and little by little the concept of praying would lose its importance and would be completely forgotten.
Obviously in order to prevent an act from fading away during different ages and eras, there needs to be an unchangeable criteria and measure that it can be judged by. For example, length is measured by the unit of centimeters and millimeters and weight is measured by the unit of kilograms and grams. The units and in other words the criteria never changes with the pass of time and this applies to praying too, meaning that there are certain criteria for prayers which are the wajib acts and the fundamental acts of prayer and among these fundamentals is saying the wajib phrases in Arabic.
2- Islam is a global religion that aims to unite all Muslims and without a language that they can understand each other through, such a goal is unachievable. Arabic, which according to the experts, is one of the most comprehensive languages that has the potential to be an international language, one which Muslims can be united by and will remain as a sign of Muslim unity, just like in other aspects of Islam, like praying towards one qibla and so forth.
3- Some may think that forcing people who aren’t good at Arabic to pray in this language is not fair because of the trouble they need to go through to learn it. In this regard one must say that learning a total of 20 words (without counting the words repeated more than once) that are used in the prayer isn’t considered difficult for those who can easily learn and use tens and hundreds of words and phrases of other languages! Add to that the fact that the simple meanings of the phrases in prayer are easy to learn and understand and all people are capable of learning the simple meaning of (بسم الله الرحمن الرحیم) and other phrases, although these phrases do have profound and deep meanings as well.
4- According to language experts, Arabic is one of the world’s most comprehensive languages that has the capability of conveying deep meanings in short and beautiful phrases.
5- It is not obligatory to perform all acts of worship in Arabic. For example, some Islamic scholars’ fatwas do not say that it is wajib for the marriage contract to be pronounced in Arabic; some great scholars like Imam Khomeini say that if a person isn't capable of pronouncing the marriage contract in Arabic and is able to get an agent (wakil) to do it in Arabic for them, it isn't necessary and he/she can do it his/herself in his/her own language. Also, it isn't necessary to do supplications and call Allah (swt) in Arabic and is okay to talk to Him in other languages (of course not in prayer) and according to the fatwa of the majority of faqihs (jurisprudents), one can pronounce all phrases of the prayer other than the wajib ones in other than Arabic, especially in the qunut (which isn't wajib). What was said of course was not meant to encourage Muslims to ignore the meanings of the phrases in prayers, for all Muslims are responsible of learning the meanings of these prayers and supplications in order to understand what he/she says while speaking to Allah (swt). This gives rise to one’s worship not being a mere formality anymore, giving it life and integrity, helping him/her rise in spirituality.