I see so the Christians are using as like we Muslims use "In Sha Allah" (If God Wills) in our normal words. However the time we use the words Amen is like when when pray, and reciting verses from the Quran, asking Allah for our needs . For example if we pray and ask god this >>>
"Guide us in the straight path, The path of those on whom THOU hast bestowed THY favours, those who have not incurred THY displeasure and those who have not gone astray"
We say Amen which means "Oh God Answer our prayer" that's why when you see Muslims praying and asking God other Muslims saying Amen meaning oh God please answer that.