A custom pack is more or less like registry for a pc.If you open a xml file you see.There are false or true like 01 or 00 in the registry.There you can add things
like show sos calls or not,techmode or not etc.You can also modify it to fit your region.As far as I know (I have alredy done it) you can modify the CDA (numbers) and add languages (allowed languages) by replacing the short writing of the eg de,fr,en,zu and place the initials of your language (as long as there is in the firmware).So find a custom pack for your mobile ,open xml,modify cda,language and everything you need and load it.

ps sometimes there are additional files in a custom file like ringtones,images etc (branded firmware).If you modify such a custom file remove the additional files and any references to them.