This topic relates to anything that includes custom language fallback enhancements prior to Sitecore 8.1 and all Sitecore supported language fallback feature from 8.1 and up.

In a multi-language solution, you can use the language fallback functionality to control which items or fields that you want to reuse content from another language, and which items and fields that you want to translate.

You specify a fallback language for the languages in your Sitecore solution and enable language fallback on the relevant items or fields. If a version does not exist in a given language, language fallback activates, and the item or field value is displayed in the fallback language instead.