This is currently the most complete localization system available for Unity.

Supports
Unity UI, Unity 2D, TextMesh Pro, NGUI, 2D ToolKit, SVG Importer, Standard Components

Localizes
Image, Text, Sounds, Fonts, Sprites, Atlases, Prefabs, TextMeshes, Dropdowns and more.

Auto Translation
Built-in support for Google Translator to automatically localize all your labels into any language.

Google Spreadsheets and CSV files
Link external Spreadsheets and any change will be automatically downloaded, even after the game is released.

Detect and Fix Errors
Parse scenes to find missing or unused translations as well as duplicated or similar terms.

Sub-Object Translations
Set Secondary terms to change not just text and images, but also which Fonts and Atlases are used per language.

Callbacks and Parameters
Translations can be modified to insert Scores, Names and Game Variables using the grammar of the language.

Plurals
Built-in rules for each language covering those that just need singular/plural forms up to those that use 6 variants.

Right to Left Languages
Correct rendering of RTL languages with support for line wrapping and alignment.

Save Memory
Create Fonts with only the characters you are using. Link your AssetBundles to add more localized content.