Since its inception on 2000-05-29, SQLite has been implemented in generic C. C was and continues to be the best language for implementing a software library like SQLite. There are no plans to recode SQLite in any other programming language anytime soon.
The reasons why C is the best language to implement SQLite include:
C is a great language for writing fast code. C is sometimes described as "portable assembly language". It enables to developers to code as close to the underlying hardware as possible while still remaining portable across platforms.
Other programming languages sometimes claim to be "as fast as C". But no other language claims to be faster than C for general-purpose programming, because none are.
Nearly all systems have the ability to call with libraries written in C. This is not true of other implementation languages.
So, for example, Android applications written in Java are able to invoke SQLite (through an adaptor). Maybe it would have been more convenient for Android if SQLite had been coded in Java as that would make the interface simpler. However, on iPhone applications are coded in Objective-C or Swift, neither of which have the ability to call libraries written in Java. Thus, SQLite would be unusable on iPhones had it been written in Java.
Libraries written in C do not have a huge run-time dependency. In its minimum configuration, SQLite requires only the following routines from the standard C library:
In a more complete build, SQLite also uses library routines like malloc() and free() and operating system interfaces for opening, reading, writing, and closing files. But even then, the number of dependencies is very small. Other "modern" language, in contrast, often require multi-megabyte runtimes loaded with thousands and thousands of interfaces.
The C language is old and boring. It is a well-known and well-understood language. This is exactly what one wants when developing a module like SQLite. Writing a small, fast, and reliable database engine is hard enough as it is without the implementation language changing out from under you with each update to the implementation language specification.