For people who don't understand the Arabic language well, what is advised for them to do while hearing Du'aa in Arabic? Is reading Qur'an and reciting supplications in another language other than Arabic considered worship and the person who does so gains the benefits of worship as he would in Arabic. Or is it just considered as reading and enlighten?

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. We ask Allaah to exalt his mention as well as that of his family and all his companions.

There is no harm on whoever is not fluent in Arabic to say 'Aameen' when hearing another person supplicating in Arabic if such a person is trustworthy, as is the case in saying 'Aameen' after hearing the opening chapter of the Qur'an being recited in prayer. As regards reciting the Qur'an in a language other than Arabic, it is not permissible either in prayer or outside the prayer according to the most preponderant opinion of the scholars, let alone ruling it as an act of worship or not. With regard to supplicating in another language than Arabic outside the prayer, it is permissible for whoever is not able to speak Arabic properly according to the most preponderant opinion. Indeed a person who supplicates in a language other than Arabic is rewarded though his supplication is not of the same excellence as the one who supplicates in Arabic. 

Finally it should be noted that Arabic is the language of the Qur'an, therefore a Muslim should learn it. Moreover, it is an obligation on him to endeavour to learn sufficient knowledge with which he performs the obligatory acts of worship, i.e. that which is not permissible in another language than Arabic like reciting the Qur'an in the prayer and so forth.  

Allaah knows best.