When he recites a verse during prayer he translates the meaning into his language in his mind

There is nothing wrong with it if you recite a verse of the
Qur’aan during your prayer and ponder its meaning in your own language,
and say that to yourself as you mentioned in the example. This comes under
the heading of pondering and thinking of the meaning. That is on condition
that you do not utter it out loud, because the one who is praying is
forbidden to speak during the prayer, based on the report narrated by
Muslim (537) from Mu‘aawiyah ibn al-Hakam al-Sulmi who said: Whilst I was
praying with the Messenger of Allaah (blessings and peace of Allah be upon
him), a man among the people sneezed and I said: Yarhamuk Allaah
(may Allah have mercy on you). The people glared at me and I said: May my
mother be bereft of me! Why are you staring at me? They started striking
their hands on their thighs, and when I realized that they were telling me
to be quiet, (I felt angry) but I kept quiet. When the Messenger of Allaah
(blessings and peace of Allah be upon him) had prayed – may my father and
mother be sacrificed for him; by Allaah I have never seen a better teacher
or better teachings before or since; he did not rebuke me, hit me or
revile me – he said: “This prayer is not the right place for any of the
people’s speech, rather it is tasbeeh, takbeer and recitation of Qur’aan,”
or words to that effect. 

And because it is not permissible to recite Qur’aan in any
language other than Arabic. See the answer to question no.
3471. 

It is permissible in naafil prayers to hold a Mus-haf with a
tafseer in the margins or footnotes, and to look at the meaning of what you
do not understand. See the answer to question no.
9505 

And Allah knows best.