When inviting a new user, you select the language in which the invitation email is sent; this language also becomes the user’s default language within the application itself. Sturely explicitly states that the user can change this language preference later, for example via their own Profile settings, meaning that the initial choice made when sending the invitation does not constitute a permanent restriction.
For organisations that operate internationally or have staff in different language regions, it makes sense to always select the new user’s native language when sending an invitation, so that they have a familiar and understandable experience from the very first email, rather than having to switch the language later on.