Getting started
Languages and URLs
Pick the languages, decide what the URLs look like, and let the language live in the address.
Choosing languages
Translations → Languages holds the list. One language is the source (the one your content is written in) and every other one is a target. There are 108 to choose from.
Start with one target language. A second is a click, but the first one is where you find out whether the tone is right, and fixing that once is cheaper than fixing it in five languages.
The language is in the URL
Every language gets its own address: either a subdirectory (example.com/nl/) or a domain of its own (example.nl). Never a cookie, never a header, never the same URL serving different text to different people.
That one decision is why page caches, CDNs and search engines cope with WPGlotty without any special configuration. Two languages are simply two URLs, so there is nothing to vary on.
The default language prefix
By default the source language has no prefix: example.com/ is the English page and example.com/nl/ is the Dutch one. You can give it one, so that English lives at example.com/en/ and the bare root redirects.
Prefixing the default is the tidier structure and the more disruptive change. Do it on day one or not at all: switching it later moves every URL you have.
Per-language settings
Each language carries its own tone of voice, formality and glossary. Dutch can be informal while German is formal, and both can be told that your product name is never translated.
Those settings have their own article, because they are where most of the quality comes from.