Add lang attribute to editor UI elements.

[Martin Kou]

Checkpoint 4.1 of WCAG: ​http://www.w3.org/TR/WCAG10/wai-pageauth.html#tech-identify-changes

The lang attribute is important because it tells screenreaders about the language it should use to read text. For example, if a screenreader assumes the page is written in English (because the webpage is written in English), yet the editor UI is being displayed in French (because browser's preferred language is French), then a screenreader would read the editor's UI labels as if it's written in English. That would mean unrecognizable speeches from the screen reader.

It should be enough to add the lang attribute to three places:

1. The container element containing the main editor UI.
2. The container elements containing dialogs.
3. The iframe documents of floating panels.

[Martin Kou]

Fixed with [3526].

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