English is a great lingua franca IMO because of its inherent decentralization and willingness to accept new words. There is no academy regulating its use, as is the case with French and Spanish.
Spanish is very willing to accept new words, and as diverse as English in terms of decentralization. Grammar doesn't make or break a lingua franca, number of speakers does, which is where English really shines.
So what does the RAE do you ask? They write grammars and compile dictionaries, describe phonology and answer people's questions on Twitter. Just like what Merriam Webster or Oxford would do, but the RAE has official backing and creates consensus among the hispanophone countries. English is a regulated language, just not officially regulated.
Some other language authorities do not take a usage-evidence-based approach to defining their dictionaries, and take into account cultural or historical concerns.
The role of regulation is mostly a legal role employed by countries that use a civil legal system. Most English speaking countries use a common law legal system and so interpretation of words is fairly fluid and subject to interpretation by the courts. Civil law does not have this kind of flexibility and room for interpretation, so courts will make use of regulatory bodies to uniformly interpret language.
This is why almost all countries with a civil law system have a regulatory body, while countries with a common law system do not.