If you are considering to start a multilanguage site, the advice I can give is that don’t bother with it.
The best way, to have a multilanguage site, is to have two sites. Two different domains with the same content so long the language is different.
As long as you are not providing a displaying site, a site usually for display purpose not for content creating purpose, you can have a multilanguage site which includes other languages in one single domain.
Your domain may look like this for multiple languages:
Each language has its own subdomain, however, this type of site is only, and solely, useful for display purpose. You don’t plan to add too much content on it, or you don’t try to update it too often.
A landing page, a company website etc. Perfect examples for multilanguage site.
So the reasons why you should get a second domain, or third, fourth domain for your website is simply because, as long as you want to monetize on it, or being professional on this website, you have to separate all of them.
Each language corresponds to each specific language-speaking region, which has its very own ecosystem.
Your search console, analytics, theme, content. So many elements you need to consider, and every language would cast a huge gap between one another.
If you want to save time and being more proficient, register for other domains for your other languages. It’ll be worth it. Specifically for blogs. You don’t want to have a multilanguage site. Just sitemap is inconvenient enough to keep you from optimizing the site.
There are ways to circumvent the multilanguage barrier, however, for time saving sake, just register for another domain, which will makes things so easier.
Another thing is, if you try to change your WordPress theme, save and backup the theme files before you change. Any modification you made on the theme files will be replaced by the new theme files.