How to Change Language on PS5 for Titles with Multi-Language Support
Instructions
Do make sure the title in question has multi-language support, otherwise as far as I know, you do need to purchase the game again.
From your PS5, Settings > System > Language, change the console language to the language of your choice for your game. Once the console has loaded into the new language, the game would also load into the language of your choice.
Changing the console language will restart the game, so please do make sure to save if there is one running in the background.
Afterthoughts
Sony could very well prevent this language support issues from happening. To take smartphone as an example, iOS sandboxed each individual app to have its own language priority list, if needed. Heck, as far as I can recall, Macintosh had language priority system built into the operating system; instead of choosing one language pack, you would select which one takes the priority so that the system would know which one to fall back to.
Odd thing is PS5 does support dual language input on its system. I take it Sony did think about multi-language support at some point, but it didn’t make it through yet. I genuinely hope Sony to create this quick and easy convenience feature for gamers, especially in this time and age where language is least of our concerns to start a new chapter in a new game.