add lang attribute for frame

[danielduan]

Issue: #2538 lang attribute missing for storybook menu frame

What I did

How to test

Is this testable with jest or storyshots?

Does this need a new example in the kitchen sink apps?

Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

[codecov]

Codecov Report

Merging #3065 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3065   +/-   ##
=======================================
  Coverage   36.78%   36.78%           
=======================================
  Files         420      420           
  Lines        9873     9873           
  Branches      824      833    +9     
=======================================
  Hits         3632     3632           
- Misses       5772     5777    +5     
+ Partials      469      464    -5

Impacted Files	Coverage Δ
addons/knobs/src/components/types/Color.js	8.1% <0%> (ø)	⬆️
...src/server/config/WatchMissingNodeModulesPlugin.js	0% <0%> (ø)	⬆️
addons/actions/src/lib/util/typeReviver.js	71.42% <0%> (ø)	⬆️
lib/core/src/client/preview/config_api.js	45.76% <0%> (ø)	⬆️
app/angular/src/server/angular-cli_config.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/routes.js	0% <0%> (ø)	⬆️
addons/a11y/src/components/WrapStory.js	61.11% <0%> (ø)	⬆️
...ents/stories_panel/stories_tree/tree_decorators.js	34.37% <0%> (ø)	⬆️
addons/knobs/src/components/types/Date/index.js	23.52% <0%> (ø)	⬆️
addons/info/src/components/makeTableComponent.js	67.46% <0%> (ø)	⬆️
... and 47 more

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