Automatic redirection is a feature that sends your visitors to the language alternative of a page they understand best. It can be enabled for the network in Network Admin/Settings/Multilingual Press. You can override that setting in each site’s extra settings in Network Admin/Sites/Edit/Multilingual Press.

Redirect network settings

Redirect site settings

The behavior is controlled by the priority value you set for each language in Network Admin/Settings/Language Manager. Use the highest value (10) for the original language, slightly lower values for the translations, depending on their quality.

MultilingualPress Language Manager
MultilingualPress Language Manager

How it works

When a visitor understands one or more languages, these preferences are sent by the web browser to the web server. Each language can get a weight, a numeric value indicating how good the visitor understands the language.
Multilingual Press will multiply your priority value with the visitor’s value for the same language and sent her to the best match, the language with the highest value.
Language sub-sets like en-ca versus en-bz are compared against the first part only if there is no better match.

For details see Language negotiation – how our redirect feature works.