Start with the market, not the plugin. Choose locale URLs, such as one folder per language (/de/, /fr/), and keep one address as the default.
Write a glossary before the first page. Product names, buttons and claims should not change halfway through.
Research search terms in the target language. A direct translation of an English keyword is often not what people search for.
Leave room in buttons and headings. German and Dutch often run longer than English, and Japanese needs different line breaking.
Add hreflang, translate the titles and descriptions, and do not rely on the browser to redirect everyone automatically.