Why a new namespace is created, when I upload a translation file?

When importing a translation file in the Control Center view, after having selected the file, you'll see a screen like this:

By default the "Target namespace" is set to the name of your file.

If you want to import the translations to a specific namespace, make sure you select the appropriate namespace:

Alternatively, you can import the file directly in the correct namespace via the Focus view or in the Global view by previously selecting the correct language and namespace.

Here you can find more information on how to delete a namespace.