Wht is the right word order?
他和你用哪种语言谈话？


"他和你用哪种语言交谈？"

'交谈' is better than '谈话' because '交谈' means 'talk to each others; while '谈话' mainly means 'speak' or 'talk'. 

You can speak or talk by yourself, but you cannot having a conversation on your own.


  学习语言的时候有什么困难？


No problem above 


  一个孩子每天多长时间应该练习语言？


"一个孩子每天应该花多长时间练习语言?"

Better add an verb '花'(spend) here 


  一个儿童可以说几种语言？


No problem above


  你哪种语言应该学习？


You are placing the [object] directly after the [subject ] 


[你][哪种语言] 


It is better to place the verb '学习'directly after the subject '你'  


"哪种语言你应该学习?" 
"你应该学习哪种语言?"



  你的父母会什么语言?


No problem here


  你未来想不想学习一种新的语言？


你将来想不想学习一种新的语言？

Should replace '未来' with '将来' for the meaning of: "in the future" because '未来' itself is a noun or an adjective for 'future'