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Your own private du'a, whether for forgiveness or anything else, can be made in any language. It is not best done in Qur'anic Arabic if you cannot speak that properly. The best is to ask in your own language, as that's what you're fluent in, and that's what you can best express the exact emotions you want to in front of Allah. Allah understands all languages, so you can ask in Moroccan Arabic. However, there are some specific du'as in Qur'anic style Arabic from the sunnah for forgiveness (such as sayyidul istighfar, below), which would be good for you to learn as the Prophet :saws: made those du'as, which have unparallelled benefits, and generally knowing Qur'anic Arabic is a good thing, as it will help you read and understand the Qur'an and du'as from the sunnah correctly.
On the authority of Shaddad ibn Aws, may Allah be pleased with him, the Prophet, :saws: said, “The chief of prayers for forgiveness is:
اللَّهُمَّ أَنْتَ رَبِّي لا إِلَهَ إِلا أَنْتَ خَلَقْتَنِي وَأَنَا عَبْدُكَ وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَيَّ وَأَبُوءُ لَكَ بِذَنْبِي فَاغْفِرْ لِي فَإِنَّهُ لا يَغْفِرُ الذُّنُوبَ إِلا أَنْتَ
'Allahumma anta Rabbee
Laa ilaaha illaa anta, Khalaqtanee
wa ana abduka, wa ana 'ala ahdika
wa wa'dika mastata'tu,
A'oodhu bika min Sharri ma sana'tu,
Aboo'u Laka bini'matika 'alaiya,
wa Aboo'u bidhanbee faghfirlee
fa'innahu la yaghfirudh-dhunooba
illaa anta.'
Translation: O Allah! You are my Cherisher. There is no deity except You. You have created me and I am Your servant and as far as possible, I abide by my solemn promise and covenant (which I made to You). I seek Your protection against the consequences of my wrongdoings. I fully acknowledge the grace You have bestowed upon me and I confess my sins. So pardon me my sins as none besides You can pardon sins.
The Prophet, :saws:, added, “If somebody recites it during the day with firm faith in it and dies on the same day before the evening, he will be from the people of Paradise and if somebody recites it at night with firm faith in it and dies before the morning he will be from the people of Paradise.” (Sahih Al-Bukhari, Volume 8, Book 75, Number 318)
It is for this reason that it has been named Sayyidul Istighfaar meaning The Chief Form of Repentance.
You can also say du'as from the sunnah and supplement them with du'as in your own language.
For anyone who wishes to say words in their own language seeking forgiveness or making du'a:
rather the one who makes du’aa’, if he is not used to speaking in grammatical Arabic, he should not force himself to do so. One of the salaf said, if a person forces himself to speak grammatical Arabic, he will not be focused in his heart.
the du’aa’ should come from the heart, and the tongue should simply follow the heart. Whoever focuses in his du’aa’ on making the words grammatically correct will have less focus in his heart. Therefore the one who is in urgent need should pray from the heart with whatever words Allaah inspires him to say, without preparing it beforehand.
"It is permissible to make du’aa’ in Arabic or in languages other than Arabic. Allaah knows the desire of the one who is making du’aa’, even if he cannot speak Arabic correctly, for He understands all languages and understands the needs of those who speak different languages."
Source of quotes
And Allah knows best.
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