Adding/Removing the language entries in the "Settings" -> "Select Locale" from Android phone
I think that a normal application should not add locations to the phone. There are special packages like MoreLocale 2 for that purpose. If you'd add a locale, you'd be responsible not only to provide localisation for your own application but for the whole system!
you have to create "values" folder in "res" folder of android's framework-res package. Name of that new folder have to contain country code of your need ie. values-pl-rPL (the rPL part is required) for polish language (for british english: en-rGB, etc), that should be enough - if not - create strings.xml file in that folder, and add there empty "resources" node.