Building a desktop/android app

hello, i am just starting my first Kotlin project, my project need to provide service for 2 clients, one that owns products and the 2nd that buys the products.
i was kind of confused what way should i go, and finally decided to go with the multiplatform way to reduce consumed time ,yet i am still confused on few different things which i hope someone can help me with.
1- what library should i use?
2-should i use Kotlin language alone in my app ?and is that even possible ? and if not than what other language should i learn to completely build my app on my own? "from front-end to back-end

thank you in advance