NGUI UILabel - can't translate text

Hello Everyone!
I have a pretty strange problem.

I added the "l2 Localize" component to NGUI UILabel.
While playing an app in Unity Editor the language is changed from the code:

LocalizationManager.CurrentLanguage = "English";

But label's text is not changing.

I noticed, that l2 localize's Target has only one option "Prefab".
I supposed it should also have a Text option, but it doesn't.
Screenshots are attached.

Please, help me to figure out, why label's text is not translating.

I'm using:
- Unity 5.6.5
- l2 localization 2.8.12
- NGUI 3.9.2.

Many thanks in advance.

Thanks a lot for your quick response.

I've manually added NGUI to "Scripting Define Symbols" and it brings the next compile errors:

It seems my version of the NGUI (3.9.2) is too old and have neither NGUIFont nor NGUIAtlas.
So, I've commented out the if-branches from LocalizeTarget_NGUI_Label.cs and LocalizeTarget_NGUI_Sprite.cs.

Then I realized somehow there is no NGUI definition in "Scripting Define Symbols".
I added it again, Unity started compiling something, shown some warnings and again NGUI disappeared from "Scripting Define Symbols". And there is still Prefab only in Targets on UILabel.
What a magic.

What is better to do?
Should I upgrade NGUI to the latest version or there is more cheaper way to fix it?
Thank you.

You saved my day! Many thanks for the advice. Now everything is working.