All prophets come and speak in the language of the people they are sent to. Allah tells us :
And We did raise among every people a Messenger, preaching: ‘Worship Allah and shun the Evil One.’ Then among them were some whom Allah guided and among them were some who became deserving of ruin. So travel through the earth, and see what was the end of those who treated the Prophets as liars!(Chapter 16 verse 37
All prophets speak the language of the people whom they are sent to and this is common sense, or how else would the people understand what they said? The Holy prophet(saw) was sent to the entire mankind but during his area the language was indeed Arabic so Allah revealed in Arabic. During the time of Ahmad(as) the languages were much more so Allah spoke to him in several languages. Is one amazed that Allah knows more then Arabic? Only an ignorant Muslim would have such a feeling that Allah is unable to speak English or Farsi. All prophets speak to the people in their languages for example when Moses(as) and Yusuf(as) were sent mainly for the Jews, they would speak in their language and to the Egyptians in their language. Ahmad(as) is basically saying that prophets have to receive revelations in their language or how would the people understand.It is indeed absurd if all of the prophets revelation was to come in a language other than what he knew but Ahmad(as) did not have this rather he had a minority of revelations in the languages he did not understand and Allah made it clear that he can indeed speak in any language he wants to. There is no room for allegations in this. Even if Ahmad(as) was unaware that Allah would reveal in other languages, Allah had shown him right away that He indeed does speak in any language He desires. Here is the context which makes it clear as he was referring to Muhammad(saw):
Ahmad(as) also states:
“This is a strange matter that some of the sentences of revelation from God do not apparently follow the man-made rules of syntax and morphology, but with slight concentration it can be brought into conformity. It is for this reason that some ignorant people have made criticisms on the Holy Qur’an based on their fictitious grammar, but all such criticisms are outrageous. Knowledge of language is known to God the All-Knowing; not anyone else. And language that changes according to place likewise changes according to time. If today’s Arabic expressions in Egypt, Mecca, Madina, Persia and others are analyzed, then these expressions are apparently uprooting all of the rules of Arabic syntax and morphology. It is possible that such expressions have been used in the past as well. So there is no obstacle in Gods revelation that some of its sentences be in line with past or present expressions (Ruhani Khazain, Volume 18, Page 436, Nuzulul Masih, page 58)
In this condition, I received a revelation of which I could only remember one phrase. Since the revelation descended at a very fast speed like the flash of lightning, the remainder could not be preserved. (Hazrat Mirza Ghulam Ahmad, al-Badr February 27 1903)
He(as) also states:
“Other than this, sometimes the revelation is being received in a foreign tongue, of which this humble one is wholly unacquainted, and then that revelation being based upon prophecy is among miraculous wonders which point towards the vast powers of Allah. Although all the words of a foreign tongue are not preserved, and because of the speed of the revelation, and being unaware of the speed of the revelation, and being unaware of the language and dialect, some deviation occurs, but there is usually less deviance in the sentences which are clear and uncomplicated. It also so happens that because of the swiftness of the revelation being received, some of the words are not preserved in memory, but when a sentence is repeated twice or thrice, then the words become well remembered. At the time of revelation, God works through his special faculty in which there is no use of external or internal means. At that time, the tongue is a tool in the hand of God. He uses it in whichever and in whatever way He chooses. Many times it happens that the words come forcefully and with haste, and sometimes as though someone takes a step with grace and dignity and then halts before taking the next step and shows his class in walking. The wisdom in adopting these two manners is so that there may be a clear difference between heavenly revelation and thoughts of the self and from Satan and so that the revelation from God may be immediately recognized by virtue of its majesty and beauty (Ruhani Khazain, volume 1 pages 570-571, Brahin-e-Ahmadiyya part 4, pages 479-480)
Ahmad(as) himself made it clear that these revelations are forgotten and sometimes a slight departure from the pronunciation. He(as) further states:
“As these revelations are in a foreign tongue and revelation comes fast, it is possible that there may be a slight departure from the pronunciation. (Ruhani Khazain Volume 22, page 317, Haqiqatul Wahi page 304)
He(as) also states:
“Thereafter, two English phrases were revealed of the correct wording of which I am still not sure on account of the speed of the revelation. They are: I love you. I shall give you a large party of Islam” [Barahin-e-Ahmadiyyah, part 4, p. 556 sub-footnote 4, Ruhani Khaza’in, vol. 1, p. 664 sub-footnote 4 See also Barahin-e-Ahmadiyyah, part 5, p. 80, Ruhani Khaza’in, vol. 21, p. 105]
He(as) also states in regards to the same issue:
“In the course of this week some phrases in English and other languages have been revealed to me…which are: [Pression, ‘Umar, Bratos or Platos] The speed of revelation left some of these words indistinct. It is not clear whether it [the last word] is Bratos or Platos. ‘Umar is an Arabic word. We need to enquire into the linguistic origin and meanings of Bratos and Pression.”(Maktubat Ahmadiyya Volume 1, Pages 68-69)