When you open Bloom for the first time or click Create New Collection, a box should appear where you can choose the type of collection.
I suspect you have chosen Source Collection, but that is normally used for creating shell books or templates in a LWC that are then made available for others to use. It sounds like it’s not a LWC, so you should choose Local Language.
With a source collection, you need to go to Settings to change the language. If you choose Local Language Collection, you will be taken through a series of steps to setup the Collection (including language #1). Once the collection is created, you can then go to the Settings option and change languages #2 and #3.
The Bloom Help menu contains information about creating a Collection and lots of other helpful information too.