How do I change iPhone language to Lithuanian?
My understanding is that Lithuanian is not one of the 31 languages supported natively by iOS. However, since iOS 8, iOS apps can be in any language, even if Apple hasn't localised it yet. 

In iOS 8 and above you can do this as follows:


Go to Settings
Tap on General
Tap on Language &amp; Region
Near the top, tap on the blue Other languages... option
In the search field just enter 'lit' to filter the list down to Lithuanian
Select the language


You may also find these links useful with what you're doing:


iOS Supported Language Codes (ISO-639)
Build Apps for the World