"Element with lang attribute has valid language tag" (de46e4): Apply to body and ignore attributes with no effect

[Jym77]

Align the "lang valid" rule with the newer "lang matches default" one by

• also applying on body itself;
• only checking lang attributes who affect some text.

Closes issue(s):

• closes "Element with lang attribute has valid language tag" (de46e4): Should it check accessible names? #1520
• closes "Element with lang attribute has valid language tag" (de46e4) fails on nested attribute #1525

Need for Final Call:
This will require a 1 week Final Call (small fixes to Applicability)

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[carlosapaduarte]

Some colons missing

Good catch

[Jym77]

Call for review ends on June 17th.

[Jym77]

Call for review ended. Merging.