When you want to add locale-specific resources in Android, you have to add the lowercase ISO-3166-1 code of the language to the resource folder\'s name. So far so good.
We can now create our "values-eu-rES" folder and have our app in Basque, Android Lollipop has made it.
The thing is, if the language is not listed in the language selection list on the device, the resources can never be used as the device can never be put in this configuration by the user.
On the other hand, if the device allows you to select Basque, it will be using this format, and your resources will be used.
Actually not all languages are documented at Android docs and many brands did your custom version of Android which may change its list, but be sure that if a device has Catalan and/or Basque suports it will use ISO standard, and the values will got rigth as it is converted with string comparision.
Maybe it would be a good choice to put a handed changer if you got that most devices do not have this support on your tests.
Many devices ship with a limited selection of languages in the settings menu, but can actually be set to any system locale using an app such as LocaleSwitch.
It should be pretty easy to test whether Basque is supported by adding a custom locale in LocaleSwitch and then adding the resource folder to see if the system loads the resources from this folder automatically.
Finally, Gingerbread 'support' for Catalan may only reference the inclusion of android.R values since 2.3, though I can't see them in the framework repository.
We can create a folder
Basque strings -> value-eu [Basque]
another folder
Catalan strings -> value-ca [Catalan]
Adding a new language in LocaleSwitch, you need to type the ISO 639-1 code. For instance, if you want to switch to basque you need to enter "eu". Afterwards, you will see the new locale as "vasco".
That makes me think that basque locale is supported by android.