In HTML, it’s good to have a lang attribute in <html>, e.g. <html lang="en">.
How is this useful?
If this is used for translation, even if the language is set to English and there are all Chinese text in the document Google Translate detects it as Chinese, not English (this means Google ignores the lang attribute).
I am quoting this from W3C:
Also a good read is Why use the language attribute?.