In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, the Fatwa Center at Islam Q & A, states:
If one can say du`aa’ well in Arabic, it is not permissible for him to make du`aa’ in any other language.
But if he is unable to make du`aa‘ in Arabic, there is no reason why he should not make du`aa‘ in his own language, so long as he starts learning Arabic in the meantime.
With regard to making du`aa’ in languages other than Arabic outside of prayer, there is nothing wrong with that, especially if that will make the him more focused in his du`aa`.
Ibn Taymiyyah said:
It is permissible to make du`aa’ in Arabic and in languages other than Arabic. Allah knows the intention of the supplicant and what he wants, no matter what language he speaks, because He hears all the voices in all different languages, asking for all kinds of needs. (Majmu` al-Fatawa, 22:488-489)
Almighty Allah knows best.