Module blocks: Language switcher and Translations

There are two main blocks providing links to other languages. They can be enabled in the administer > blocks page.
Both are named 'Languages' and look the same, displaying a collection of flags and language names, but the behaviour is quite different.

Language switcher

This one is provided by i18n.module. It is a simple language switcher and knows nothing about translations.

Translations

This one is provided by translation.module. It is a language switcher that also knows about translation relationships, providing the right link to the translation page when available.

You have to enable one of them depending on the behaviour you need. Having both of them enabled may be confusing so better choose only one.