Ubuntu – What software exists for learning the English language in Ubuntu

Taken from help.ubuntu.com

The LANGUAGE priority list

The "LANGUAGE" environment variable, which is set by the GUIs more often than not on 
Ubuntu desktops, controls language for message and menu display for GNU compatible 
applications. For such applications it overrides whatever locale names are set in 
"LANG" and "LC_MESSAGES".

Unlike "LANG" and "LC_*", "LANGUAGE" should not be assigned a complete locale name 
including the encoding part (e.g. ".UTF-8"). Instead "LANGUAGE" should contain a 
colon separated priority list of language codes, for instance "es:de:en". A single 
language code is also correct.

As lame as a link answer is, this is probably what you want.

The post referenced in the link above explains that if you want to change the language that appears on the command-line then you need to set both LANG and LANGUAGE accordingly in either /etc/default/locale or ~/.pam_environment.

Since you've already tried setting LANGUAGE in /etc/default/locale, I'd try setting it in ~/.pam_environment since that's a user config file and its contents should overwrite anything set in the global environment.

EDIT:

I went ahead and tried duplicating the problem that you described by setting LANGUAGE in /etc/default/locale to "en:ru:en" logging out and then logging back in. It worked. Error messages were then displayed in Russian, at least for the root account (which was the account I was testing this on). Changing the value of LANGUAGE to "en" fixed it.

Due to a bug in software-center/ubuntu-software (since 2009) it's necessary to get the apt cache in english once in order to be able to start the software in english using the LANGUAGE variable as usual as pointed out by user Rinzwind:

sudo env LANGUAGE=en apt-get update && sudo env LANGUAGE=en ubuntu-software

starts the software in english.