Hi,
The function `org-babel-execute-src-block' will emit an error message
if the language is not loaded.

Before the error message, the patch will give a try to load the
associated babel language.

Why does the language exist in the `lang org-babel-load-languages' but
not be loaded?
Because the `org-babel-load-languages' are defined as a customer
variable with  :set 'org-babel-do-load-languages,  the ":set" function
can only be triggered with function `custom-set-variables`.
While setq/cl-pushnew `org-babel-load-languages' won't trigger the
`:set' function, then the error message will be displayed if I run the
code in the org file.

Please help review that patch. Thanks. Regards