It is established in Islam how exactly acts of worship are to be performed. This is known in the Shari` ah as tawqifi. Hence, Prayer is not to be offered in a language other than Arabic.
But some scholars are of the opinion that newly converted Muslims who do not know Arabic may recite Surat Al-Fatihah as translated into their mother tongue during Prayer, and may also invoke Almighty Allah in their native language. But they must seek to learn Arabic as soon as possible so that they
can perform the rituals of Islam properly. But it is to be kept in mind that this is an exception for newly converted Muslims who do not know Arabic.

Dr. Nasr Farid Wasel, former mufti of Egypt and member of the Islamic Research Academy at Al-Azhar, states: Almighty Allah has ordained Muslims to recite the Qur’an according to its prescribed rules. He Almighty says: [And recite the Qur’an as it ought to be recited] (Al-Muzzammil 73:4).

This is to say, the Qur’an ought to be recited with complete submission to Almighty Allah and deep contemplation of its meanings, as well as with thorough adherence to the prescribed rules of its recitation, such as which vocal organs to use to articulate each letter, etc.
Hence, the Qur’an is not be recited [during prayer] in a way that may change the rules of its recitation or in a language other than Arabic.

Moreover, Dr. Abdel-Azim Al-Mat`ani, a professor of post-graduate studies at Al-Azhar University, adds: An exact translation of the Arabic Qur’anic text in another language is impossible; no language can ever imitate the diction or the style of the language of the Qur’an or even a verse of it. What is only possible in this regard is to translate the meanings of the Qur’an and explain it in another language.

In addition, the late prominent Muslim scholar Sheikh Ahmad Ash-Sharabasi, professor of `aqeedah and philosophy at Al-Azhar University, stated: Some jurists are of the opinion that it is obligatory upon Muslims to perform Prayer in Arabic. But some schools of jurisprudence say that if a non-Arab person is newly converted to Islam and does not know Arabic, he may recite Surat Al-Fatihah during Prayer as translated in his mother tongue. Allah Almighty says: [And of His signs is the creation of the heavens and the earth, and the difference of your languages and colors. Lo! Herein indeed are portents for men of knowledge] (Ar-Rum 30:22).
However, scholars see that non-Arab Muslims who do not know Arabic must learn it so that they can perform Prayer as prescribed by Almighty Allah and understand what they say during Prayer. Imam Ash-Shafi`i regards this as an obligation upon every Muslim person.

Sheikh `Atiyyah Saqr, former head of Al-Azhar Fatwa Committee, further states: It is established that reciting Surat Al-Fatihah during Prayer is one of its conditions, without which Prayer would be regarded as invalid. The Prophet (peace and blessings be upon him) said: “He who does not recite Surat Al-Fatihah during Prayer, his Prayer is invalid.” In addition, it is an act of sunnah to recite some other verses from the Qur’an after reciting Al-Fatihah during the first two rak`ahs.

The majority of scholars, including the Maliki school, the Shafi`i school, and the Hanbali school, are of the opinion that Al-Fatihah must be recited in Arabic during Prayer. Hence, according to them, it is not permissible for Muslims who cannot speak Arabic to recite Al-Fatihah as translated into another language; according to them, the Prayer of one who does so will be invalid.
An-Nawawi said in his book Al-Majmu`: “It is unanimously agreed upon that the various translations of the Qur’an cannot be designated as the Qur’an itself. For instance, no one can say that one who is reading the meanings of the Qur’an as translated into the Indian language is articulating the text of the Qur’an itself. It is unreasonable to believe something like that. If one cannot say that the translation of a poem of a certain poet represents exactly the original text of the poem, we should not, with all the greater reason, say so with the case of the translation of the Qur’an. Besides, translations of the Qur’an will never be able to represent the unique diction and style that characterize the language of the Qur’an.”
However, Imam Abu Hanifah was reported to have said that it is permissible for one to recite the Qur’an during Prayer as translated into another language whether one can speak Arabic or not. Abu Hanifah cited as evidence for this opinion some verses that do not indicate directly what he held in that respect.
But knowledgeable researchers in jurisprudence say that Abu Hanifah changed his opinion in that regard later and said that it is not permissible for one to recite the Qur’an during Prayer in a language other than Arabic, unless one is unable to speak Arabic.
Hence, the Maliki, Shafi`i, and Hanbali scholars say that he who recites the Qur’an during Prayer in a language other than Arabic, even if he does not know Arabic, has his Prayer invalid, while the Hanafi scholars hold that he who is unable to speak Arabic can recite the Qur’an as translated in his mother tongue during Prayer, for he in this case is like the unlettered person who cannot recite the Qur’an, and thus given a dispensation to, instead, say dhikr however he knows.

Finally, the late prominent scholar Sheikh Mustafa Az-Zarqa concludes: There are many reasons that dictate that Prayer be performed in Arabic.

First, metaphysically speaking, a native language of a people is called a mother tongue; this is a figurative description depicting languages as what mothers speak. In this sense, let’s contemplate Allah’s words: [The Prophet is closer to the believers than their selves, and his wives are (as) their mothers] (Al-Ahzab 33:6)
This verse describes the Prophet’s wives as the Muslims’ mothers, which, following the metaphysical reasoning presented above, entails that Muslims are to speak in their mothers’ tongue. Hence, since it is well-known that all the Prophet’s wives spoke Arabic, Muslims are also to speak Arabic. This entails, in turn, that all Muslims are to perform Prayer in their main mother tongue, that is, Arabic.
Second, if this reason is not convincing enough, let’s contemplate another deeper one: according to the Islamic belief, the Qur’an is Almighty Allah’s words, and reciting these very words is a kind of worship that brings one closer to Almighty Allah. Hence, taking into account that Almighty Allah’s words were revealed to the Prophet (peace and blessings be upon him) in Arabic, when a believer recites the Qur’an, especially in Prayer, he spiritually meditates upon the sacred words of Almighty Allah Himself. No matter how precise the translations of the Qur’an are, they will never be more than human handling of the original divine text, and hence, reciting the Qur’an as translated into another language will not bring the believers the spiritual worship they aspire to while praying in Arabic.
Third, a distinction should be made between prayer [du`aa’] in its general meaning that refers to one’s invoking Almighty Allah, and Prayer [salah] in its specific meaning that refers to an act of worship. Both meanings are somehow different. When one is praying to Almighty Allah in the sense of invoking Him, one is free to express oneself in any language and in any way one wants. This is because prayer in this sense is a special personal relationship between one and one’s Creator.
As for Prayer as an act of worship, it is mainly to be performed in congregation as prescribed by the teachings of Islam. This requires that one follow the imam during Prayer. It is right that one may perform Prayer individually, but it is recommendable to offer it in congregation.
Fourth, had Islam been a religion sent to a certain people or ethnic group, there would have been no problem that Prayer be performed in the language of this people or ethnic group. But Islam is a universal religion whose followers belong to many different places of the world, and thus, speak different languages and dialects. Moreover, Islam is spreading more and more all over the world. [This further dictates that there be a united form of this essential act of worship.]
Fifth, Islam is the only religion now in the world whose teachings and rulings are maintained in their original form exactly as revealed by Almighty Allah in His Book, the Qur’an. Other religions, such as Christianity and Judaism, have only translations of the original text of their books or, at most, separate fragments of these books. How fortunate Muslims are to be the only exception in this respect, being the only people to have a divine book in its original form.
Sixth, though the Qur’an is revealed in prose, it has a unique diction, including expressive rhythm of words, rhyming style, and eloquence of expression. The deletion of a word from it or the addition of a word to it would make it lose such unique structure.
Seventh, in my point of view, Muslims would not show the translated forms of the Qur’an such reverence they have for the original divine text of the Qur’an. This is because translations of the Qur’an represent the dictions of certain human writers, while the original text of the Qur’an is of a divine infallible source.
Finally, some scholars say that the eminent jurist Imam Abu Hanifah was of the opinion that it is permissible for one to recite the Qur’an as translated in other languages during Prayer. But those scholars mention only one aspect of the picture, disregarding that Imam Abu Hanifah changed his mind in that regard later and followed the viewpoint of the other jurists. (This is mentioned in the books of the Hanifi school, such as Al-Hidayah by Al-Mirghinani, Ad-Durr Al-Mukhtar by Al-Haskafi, Hashiyat Rad Al-Mihtar.) Imam Abu Hanifah believed that, in ordinary cases, it is not permissible for one to recite the Qur’an during Prayer in a language other than Arabic.
However, there is an exception in this regard for non-Arab people who have newly embraced Islam. Once a person embraces Islam, he must perform the prescribed five Prayers daily, which requires him to recite some verses from the Qur’an by heart. But if this person does not know Arabic, he may have a dispensation to recite the Qur’an during Prayer as translated in his native language until he learns enough to be able to recite the Qur’an during Prayer in Arabic.
There is a telling precedent in this respect set by the great companion Salman the Persian (may Allah be pleased with him). With a dispensation from the Prophet (peace and blessings be upon him), he translated Surat Al-Fatihah into the Persian language and sent the translation to his fellow Persian people who embraced Islam, so that they could recite it during Prayer. The Persian believers used this translation in Prayer until they managed to learn the Qur’an in Arabic. (Reported in Taj Ash-Shari`ah and An-Nihayah: Hashiyat Al-Hidayah.)
Based on the above, every Muslim is required to learn Arabic so that he/she can recite the Qur’an as it has been revealed in the mother tongue of the Prophet (peace and blessings be upon him). Hence, Islam remains the perpetual source for Arabic.