How can I Change the store language after switching the store

You can set Locale as per store view.

Admin >> Store >> Configuration >> Select your store from store drop down on top left corner >> Locale Options >> Set your language for store.

Admin provide to Add Product as well as category data in different language as per store view.

Here for Product : Admin >> Products >> Catalog >> Open Product >> Select store view

Here for category : Admin >> Products >> Category >> Select Category >> Select store view

Note : To convert into different language you must need to install language pack into Magento as per store view.

Create CSV in Below format :

sku     | store_view_code    | attribute_set_code   | name

sample  | fr                 | Default              | sample in franch

Add your 1000 products in this format with store_view_code and import products from System >> Import >> Products