Re-Writing Content into Different Languages Is OK
Translating content into different languages, or adding additional information is OK. Auto-translation, or just swapping out individual words is not OK.
Use a Single Language per Page
Multiple languages on the same page makes it harder for Google to understand when a page is relevant to a particular audience so John recommends using a single language per page, and use hreflang to connect the pages.
Use Separate Pages per Language
It’s best to use a separate page for each language than combine multiple languages on a single page.
Hreflang Languages Must Match the Target Page Content
If the languages used in your hreflang tags doesn’t match the target content, they will probably be ignored.
Google Ignores Language Tags
Google doesn’t use any language HTML tags to detect the language of page, they use the text to detect it. They are still used by Bing and translation services.
Notranslate Meta Tag Prevents Translation Link Showing In Search Results
Google may show a translate link in search results if part of your page contains some foreign language text on the page. You can use the notranslate meta tag to prevent this, but it also prevents Chrome translate function from working automatically.
Google Ignores Incorrect Language Tags
If you use the wrong language tag, Google will just ignore it and it won’t impact search results.
Hreflanged Pages in the Same Language Don’t Need to Be Duplicates
If you connect pages together with hreflang which are in the same language e.g. UK/US/Australia, they don’t need to be duplicates. They are allowed quite a lot of variation as if they were different languages.