Please (Plis,) help me a little.

We spent two weeks in a small village in/on a mountains.

You can use both options: 

We spent two weeks in a small village in the mountains.

You can us this phrase when you spend the time in a village between a mountain range (such at the Alps, Andes).

We spent two weeks in a small village on a mountain (s)

You can use this phrase if you spend the time in a village on the side of a single mountain.

Please, help me a little. Thanks

We spent two weeks in a small village in/on a the mountains.

Please help me a little.

We spent two weeks in a small village, in the mountains.