With the latest SWP 8.1.0 on UC540, you can install up to 6 locales (these are the ZIP files you found) using CCA 3.0 Maintenance drawer, SW Upgrade: UC500 utility, which allows the locale update.
I did one today for Italian.
You can have one Active and one alternate language defined at any time, but all the phones use the ACTIVE one configured using Configure Drawer: telephony> System> Region
While we are on a path to support phone specific locales (some english, some spanish, etc...) we dont support this today.