Okay there are few approaches to this question and in order to fully explain it, we need to understand something about scholarship in general. If a proper academic scholar of Islam tells me that its not quite possible to properly understand Islam unless you can read and contextualize 7th century Quraishi Arabic then I'm likely to agree with them. Would you take a scholar of Japanese or Chinese literature seriously if they were not able to properly understand and contextualize the source material as it was written? No you probably wouldn't. However, from the Muslim perspective, the Quran isn't meant to be a dusty book on the shelf that only academics can understand.

The vast, vast, majority of Muslims don't speak Arabic. And even the ones that do speak Arabic don't speak classical Arabic. After all, Modern Egyptian or Syrian Arabic is about as similar to 7th century Quraishi Arabic as Modern American English is to Victorian English. People convert to Islam after having read the English translation, and so they're clearly seeing something in the English translation. If the English translation can capture enough of the essence of the Quran that someone can convert after having read it, then why can I not criticize such a translation? The conversation or debate usually devolves to the point of someone saying "yea well you don't understand Arabic so you automatically lose." Okay? Please tell me what it is that I'm missing from reading the translation in English. Is it some sort of sublime poetic beauty that only exists in Arabic? I can concede that point, but poetic beauty is not any sort of indication of divine authorship. And on top of that, there are many authors who have written works (in Arabic and otherwise) that translate quite beautifully to English. Why on earth would an all powerful God be so impotent as to not be able to do such as well?

In addition, why don't we carry this line of reasoning to its logical conclusion. If I am not allowed to criticize to the Quran unless I know Arabic, then how can a Muslim criticize Christianity without having read the Bible in its original Aramaic/Greek? How can a Muslim criticize Judaism without having read the Talmud/Tanakh/Torah in their original language? How can a Muslim criticize Hinduism, Jainism, Sikhism, or any other religion without having first learned their language of revelation?

Hopefully by now its becoming to dawn on you that "but you don't know Arabic!" is not really an argument at all. Its a tactic to make you shut up. Its a tactic to make you doubt your understanding of the scripture. Its a tactic to end the debate without saying anything of substance. Arabic isn't anymore of a special snowflake language than any of the other languages, and to use it to argue that the Quran can't be translated is admitting that the Quran is written so arcanely, so humanly, so poorly, that it would take years of study to properly understand a text that is supposed to be used by the lay person as a guide to their life. Sunni Islam was doing Sola Scriptura and the other Solas well before the Protestant reformers.