Google's John Mueller said that while GoogleBot generally crawls documents "without an accept-language header" it is not a good idea to use the user-agent language setting to serve content.
This is in response to a webmaster who asked about using it. I guess this can cause issues for GoogleBot and maybe users themselves?
Going the user-agent route with this language setting is "generally a bad idea" John said.
Here are the tweets:
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) August 8, 2017
Forum discussion at Twitter.