It's does say their Imam (their leader),so which nation had their different Imam,for exemple Muslims had prophet Muhammad(pbuh),Christians had Jesus(pbuh) . Jews(pbuh) had Moses(pbuh)....etc
It's clear that Mahdi never ever mentionned in Quran,linked to that verse is mistake,Jesus(pbuh) return never mentionned in Quran.
Mutishbihat in some verse not in every verse. not in these ones.
It's does not mean that you insert events or personalities does not exist in Quran.
Well-grounded is not degree that premit to some scholars to invent something not exist in Quran,or use fake hadithes as proof.
I think you are using inaccurate translation.
The word 'nation', is not in that verse.
In Quran there two words, which are translated as people by Translators: Ummah and Ghom.
Allah is specific in using words. Every word used in Quran has a reason.
The word Ummah is often translated as a Nation of a Religion. For example Muslims are a nation. Christians are another Nation. A Muslim from Islam Nation can be black, white, European...Arab.
But! The other word is Ghom . It is different. It Ghom is best understood as a particular tribe or people. For example Arabs is a Ghom. Ghom of Arabs.
Muhammad had appeared among the ghom of Arabs. In Quran, Allah says, if Arabs do not not stay on the right path, God will change them with another Ghom. Do you know which verse?
The idea is in Quran. Moses appeared among the Ghom of jews. God had chosen them before Arabs. But they did not follow God's guidance. Therefore God changed them with Arabs.
Now Quran says, Allah change Arabs. Hadithes from prophet says, this verse is an allusion to Persians. Mahdi appears among Persian according to Islamic Tradition from Muhammad and Imams (both sunni and shia hadithes).
Now, the verse which talks about 'He shall call all people of tribe (ghom), to the Imam of their time', is in parallel with the other verse which says, God changes the Arabs. It means Imam Mahdi appears among the all people (ghom) of Persia, and calls them to Book of God. This is how arabs are changed with another ghom (persians).
If this verse was talking about nations, it would use the word Ummah. Not ghom.
All i say are based on Traditions of Prophet and imams. I am not making up Tafseer or use tafseer of ordinary men who think they are true scholars my friend.