Buying Premium just adds all DLCs to your account and the Battlepacks and installs the DLCs.
You can't change the language with that, sorry.
But you can change your language with this guide:
1. Press Windows + R.
2. Enter "regedit".
3. Go to HKEY_LOCAL_MACHINE > SOFTWARE > Wow6432Node(If you don't find this, skip it) > Origin Games. If you have 32 bit OS, go to HKEY_LOCAL_MACHINE > SOFTWARE > EA Games > Battlefield 4.
4. Go through all the numbers, and check the DisplayName variable of them all. If it includes Battlefield 4™ in it, and change the Locale variable of them to es_ES.
If I helped you, feel free to press the XP button in the corner of this post and if this fixed your problem, press the Accept as Solution button.