I have two questions:

1. Will Imam Mehdi (aj) be able to speak in all languages in order to spread his message or will he only speak in Arabic?

2. When we visit the shrines of Prophet Muhammad (saw) and Ahlul Bayt (as) and speak to them, can they understand me if I speak in Urdu or English for example instead of Arabic?

1- Yes, he will be able to deliver his message in all languages.
2- You can speak any language.