There are two completely different approaches. The pros and cons are not completely obvious, but the good news is that both can be got working quickly.
The first is to put marked text in your different target languages directly in your posts and pages, and then use a software switch to just display the text in the language your user has selected. This gives you complete control over the quality of the translations offered.
How you produce the translation is up to you - you could use machine translation or hire human translators. But the only translation text that will appear is exactly what you put on your website. So for things like technical or legal information you want to make available in several languages - and which has to be correct, this is the best approach.
It’s also worth mentioning that you don’t have to translate your whole site in one go. If there aren’t alternative texts available in the language your visitor has selected the page will simply be displayed in the default language - usually the first language you have put on that page.
The second approach is to be use machine translation and be up-front about it. Provide a Google or Bing Translate box for your visitors to use. This may seem old-fashioned, because users can now easily get sites translated themselves at the browser level by using Chrome, Vivaldi or a browser extension. But your site may attract users that don’t know that.
Unfortunately Google has stopped making its traditional website translate widget available to new users - arguing it’s now up to users to use browsers properly (i.e. get Chrome!) But old Google widget code still seems to work, and Bing does provide new users with web widgets. Try bing dot com/widget/translator
The obvious con of this is that you have no control over the translation quality. But on the plus side you do get to offer support in a very large number of languages.
A less obvious disadvantage with a Google or Bing translate box is that the foreign language text isn’t on YOUR website, so you get no SEO advantage. By contrast if you use some version of the software switch approach search crawlers should find the various language versions of your article, because all these additional words are actually on your site where you put them.