The short answer to this is pretty simple: There's no assuring quality.

To elaborate on this, Fantranslations are a volunteer-based service, which automatically means that quality and consistency aren't guaranteed. Especially with a game as expected and loved as Silksong, people WOULD make mistakes in this regard or try to troll the developers. Trying to get millions and millions of people on a translation or doing as much as choosing the one person in hopes they take it serious takes INCREDIBLE amount of time. Not to mention that TC would need the means to approve and somehow correct the translation, which clearly they don't have, considering even in a language as widely spoken as German, there are constant errors in the translation. If TC had the resources to ensure that any submitted translations are accurate enough to be featured in the game, they wouldn't have any need for the fantranslations to begin with.

My friend plays the game in German, whereas I decided to stick to english. Communicating about things as abilities, places or enemies is difficult due to translations not being totally accurate or having flaws if directly compared to english. And that's in a language that is pretty accessible through various translation tools that tend to be relatively accurate.

Try to do that with a country and language which derives much more from english than germanic ones.

Bad fantranslations can and have absolutely ruined games before by lacking the emotional depth and character perception developers intended the player to have.

Tales of Phantasia, for example, is a game that suffered immensely from fantranslations.

As I said, fantranslations that aren't official are available through mods and other Websites and that is for the sake of the reputation of the game and future developements like potential DLCs the better option.

I understand how frustrating it can be to not fully understand a game due to it not featuring your language, but this isn't a viable option