It is undesirable for a person offering Prayer behind an Imam to hold or look at a translation of the Qur’an, even with the aim of understanding the Qur’an being recited.
The scholars (may Allah have mercy upon them) stated that it is disliked for a praying person to look at any script other than the Qur’an. An-Nawawi (may Allah have mercy upon him) said: “If a praying person looks at a script other than the Qur’an and recites it in his heart, his prayer is not invalidated but this is disliked.” This is also the view of Imam Ash-Shafi`i (may Allah have mercy upon him).
It is known that the translation of the Qur’an is not considered the Qur’an, because it just an interpretation of the text in another language. So, it is disliked for a person offering Prayer behind an Imam to look at the translation during the prayer. Also, reading the translation of the Qur’an during the prayer requires turning the pages. Yet, this implies missing important acts, such as tranquility and placing the right hand over the left one on one’s chest.
Moreover, a group of scholars are of the view that it is disliked for a praying person to hold a Mus-haf (Arabic copy of the Qur’an) and read from it during the Prayer. Some of them even opine that doing so invalidates the prayer. So, what about holding a book other than the Qur’an?
To conclude, it is undesirable for a Muslim led by Imam in prayer to hold or look at a translation of the Qur’an during the prayer, even under the pretext of understanding what is being recited.
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Source: islamweb.net.