Allow the language ini path to be overridden for translation testing purposes.

Related to https://github.com/hrydgard/native/pull/196, so please accept that first if this is to be accepted too.

Currently we have to keep updating lang and waiting for or compiling new Android/iOS builds before being able to test translations, which is a hassle, so if we let the user override the built-in language files with those in g_Config.memCardDirectory/PSP/SYSTEM/lang, and the problem should be mostly relieved. :)