Language::isValidBuiltInCode() should not accept uppercase input

The results of this function are used to decide whether a code is
valid for loading an i18n file without any further normalization.

Partially reverts 93348f3 which made the regular expression case-
insensitive. Per IRC discussion, language codes should always be
lowercase and it's up to callers to deal with that.

Change-Id: I8975c3374a37935080d9f7eca6a602e32f67a87b
Merge "Language::isValidBuiltInCode() should not accept uppercase input"