All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
When a person pronounces the Shahaadah in Arabic or English or his own language while intending to embrace Islam, then his conversion is valid and he is declared a Muslim even if he fails to pronounce it properly if he is a non-Arab or is not able to pronounce it properly.
He is not obliged to repeat the Shahaadah if he learns its proper pronunciation afterwards. This is not like the case in which one mispronounces the verses of the Quran in the prayer because the objective of pronouncing the Shahaadah is to declare one's conviction and faith in Allaah, and this objective is met in this case.
"If a non-Muslim wishes to embrace Islam and he does not know Arabic, then he may say the Shahaadah in his own language and he becomes a Muslim without any difference of opinion among scholars regarding that. If he masters Arabic, is his conversion to Islam valid if he says the Shahaadah in other than Arabic? There are two scholarly views of the Shaafiʻis, the preponderant of which holds that he is declared a Muslim because he holds the conviction (that Allaah is the only true God). This is the correct view and the other scholarly view is not strong..." [Sharh Saheeh Muslim and Al-Majmooʻ by An-Nawawi]
The Shaafiʻi scholar Al-Maawardi wrote, "The purpose of the proclamation of the Shahaadah is to declare the conviction (harbored in the heart) that Allaah is the only deity worthy of worship and that Muhammad is His Messenger, and the declaration of faith, in this sense, may be done in Arabic or Persian alike..." [Al-Haawi]
Allaah knows best.