The Crawl's default language, notably not the one the containment interface is written in.
The Containment Interface gives Carl his first experience of text that is not Syndicate Standard. He can read the words and understand the numbers, but there is a fractional moment before it translates — a lag he has never seen before in the Crawl's normally seamless localization.
The anomaly makes him guess at the interface's origin: the The Valtay home language, or whatever tongue Paulie spoke. It also feeds his conviction that someone outside the Syndicate built the menus, and therefore that the choice between The Fuse and Disconnect Containment might be a trap laid by hands he cannot identify.
The linguistic baseline whose absence tips Carl off that the interface is foreign-built.