Drupal Content Translation or Entity Translation and SEO - What you should know

For content translations, the rule of thumb is that focus on SEO of English version then get a good translator to research keywords and translate the content to the target language and work with them to optimize it again in the target language.

1. That comes to adding Entity translations (this is recommended since D8 upgrade will be easier) or i18n module first to Drupal project. Those two modules can handle the content translations very well in Drupal7 so either works and have many configuration options.
2. Drupal core is designed to use multiple domains and since the domain names or sub-domains are used to determine search results ranking it is recommend to use  a sub-domain ( jp.example.com) or full domain ( example.jp) for target language both have similar effect.
3. Using a consistent URLs schema for multilingual content will help, so for example use a single url for content and let Drupal decide and handle the redirects for target language.
4. Images and files can be localized so use Entity translation module or any other module to translate images and files as necessary.
5. Making sure enable `hreflang` attribute for pages will help, this attribute specifies the base language.
6. Adding multilingual site maps also will help.

That's pretty much it, if you happen to know any please comment below.