Update 'Language' section text on Settings page

Background / User story

A/B user testing proved that users are confused by the 'Language' description text. It isn't immediately clear that they have to add a language in order to block ads in that particular language.

What to change

• Design: N/A
• Research: N/A
• Spec: eyeo/specs/spec!337
• Legal: N/A
• Development: Change text from Optimize Adblock Plus for the language(s) you typically browse websites in. to Add or remove languages that you want Adblock Plus to block ads in.

Hints for testers

None.

Hints for translators

• Changed description and text of string options_language_description.