On Twitter, Google’s John Mueller said that it was a bad idea to server content based on the user-agent language setting.
Most desktop browsers have settings that allow you to check or change the language preference settings. These tell the server what language you prefer for pages and resources that it sends you (separately from the language of the browser user interface). This setting will be used in the user-agent information.
Unfortunately, John Mueller did not explain why it was not a good idea to server content based on the user-agent language:
AFAIK we mostly crawl without an accept-language header. Serving content by user-agent language setting is generally a bad idea.— John ☆.o(≧▽≦)o.☆ (@JohnMu) 8. August 2017