add for every used language in your project a startpage with type menu-link. name this added page in the official language code like DE, EN, ES, RU etc (what ever you need).
go then to the page settings from every added menu-link page and set the language from this page to the page name, DE has language = german (DE), EN has language english (EN) etc
no go to your existed pages into the page settings from every page in the main level and set the parent page to this language, what you need
Example: i 've a single language project in english and will change it to a multi-lingual project with english and german
i add a page with type Menu-Link and language EN
normalize all my existed pages in the single language project has the parent == none, here i have to change the parent now to EN (my page with the name = EN)
for all other languages.... be sure, that the start page from every language tree has the correct language in page settings before you add more sub pages in this language tree. Same prpblem, if you use the tool page_cloner - check the language from every copied file