I'm using Shiny with Python, and I'm not sure how to add a "lang" entry to the <html> element enclosing the page. One alternative might be to use ui.head_content() to add some metadata to the <head> section, but I'm not sure how to get e.g. <meta http-equiv="content-language" content="en"> via that routine (it expects a TagAttrValue - I'm not sure what the appropriate value is for http-equiv).