AlphaElwedritsch English has long been the international standard in all technical matters.
That does not necessitate that all users are proficient in it. English language proficiency varies profoundly across regions of the Earth. I'm a bit confused as to the usage of "technical matters". I find it a broad term. If we are to stay on the topic of Android devices specifically, then widely used Android variants, their help pages and stock apps are all translated into many languages, and it's not at all necessary to be proficient in English in order to use an Android device.
AlphaElwedritsch Moreover, if someone decides to use GOS on their device, they must have already spent a lot of time on it and this information is mostly only available in English
Firstly, it is not necessary to spend a lot of time to learn GrapheneOS in order to use it effectively. That is, if you can read and understand the English language, using GrapheneOS is arguably easy.
Secondly, people can hear about new operating systems by word of mouth, in their native language, from people who already know English.
Thirdly, language proficiency is not an either/or: people may know enough English to mostly comprehend what GrapheneOS is about. They could use a translation tool to translate the webpages, but that is not practicable from within an OS' settings menus and apps. It's also prone to grave mistranslations.
One can imagine that official GrapheneOS devices will, in the future, be available for sale across different regions of Earth. In such a bright future scenario, would it not be beneficial (although, strictly speaking, not necessary) for users to enjoy their devices in their native language? At the very least, it would help boost sales.
I hope it will come to pass one day. However, I do respect that providing translations for GrapheneOS – or any other serious OS with limited resources – is extremely challenging, for both technical and organizational reasons.