I tried to reproduce your problem in Chrome but I couldn't.
I am using 38.0.2125.104. So perhaps upgrading Chrome could help?
Just noticed this from the screen shot given that the title of the window is not translated.
If you also have this and are using English (see inc_init_tinymce.tpl in /templates_internal then possibly your /editor_wysi files are inconsistent.
You could try to check them or (if you have no customization done) just re-upload the files.
Quality is in the detail of things............
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