Choose the primary language - based on the majority of your content is written in. For example - if your page is predominantly written in English then your primary language should be set as English.
See - https://www.nomensa.com/blog/7-tips-for-multi-lingual-website-accessibility - It shows you Multi-lingual Website Accessibility - 7 Tips and Techniques.
Check with this - https://w3-lab.com/what-is-a-multilingual-website/ - It shows you WHAT IS A MULTILINGUAL WEBSITE AND HOW TO BUILD IT – WITH EXAMPLES
Check with this - https://weglot.com/9-tips-for-designing-a-multi-language-website/ - It shows you 9 tips for designing a multi language websites with more detailed .
See - https://www.wpbeginner.com/beginners-guide/how-to-easily-create-a-multilingual-wordpress-site/ - It shows you steps to Easily Create a Multilingual WordPress Site with detailed .
Hope this Helps !