You should now select those languages that you will need to work with in the new system. It is possible to input and output English text in any Linux system. To do this, there is no need to select «English» at this point.
For each language selected, all components needed to enter and display text in this language will be installed in the system. This includes system messages and program interfaces, date formats, currency symbols, fonts, encodings and keyboard layouts.1 You should only select «English» at this stage of the installation if you need to see system messages, dates and other expressions formed according to rules used in Great Britain or the USA.
One of the languages installed in the system is considered the system's main language. This language is specified first in the list of the system's available languages and is used by default by the majority of programs. A language chosen as the main one is that language in which the installation of the system is carried out. After the installation the main language can be changed at any time to one of the other languages installed in the system. All of the system's other languages can be referred to as additional languages.
Next step: Keyboard layout for the main language
One level up: Second Installation Stage
1It is more appropriate to talk about locales, rather than languages, in this context. A locale is determined by a combination of a language and a country (for instance, en_US means US English, en_GB British English, ru_RU Russian in Russia, etc.). The «system» English language, always present in Linux, corresponds to a special locale, called C or POSIX. The locale determines which standard system concepts and messages should be formed according to rules used for this language/country combination. For each of the languages selected all locales that exist for it will be installed.