What's the purpose?
If it is to enable more functionality and all that, it might be oddly worth considering adopting things from other editors, (as they have adopted much form TextMate (tmbundles and grammars)).
JavaScript/TypeScript are obvious choices there, to enable access to more extensions and existing LSPs, they are also implemented in runtimes that are… implemented in C/C++ (and for WebKit and JavaScriptCore Objective-C/Objective-C++)
But having things available in Python/PyObjC might be a huge draw.
Swift is definitely a good consideration, but Objective-C/
Objective-C++/C/C++ are all great tools as well and they honestly integrate better with other languages and runtimes in general.
Ruby is still a great tool, and as such Crystal might be a worthy consideration also.
Currently TextMate is written in mostly C++ and Objective-C++ (not sure if there’s any pure Objective-C). If one would contribute something larger to TextMate, something that makes sense to put in a new file, can we use something other? The obvious candidate is Swift. The major downside of using Swift is that it doesn’t support interoperability with C++ (but it looks like it might in the future [1]).
As a wildcard, I can mention the D programming language [2]. It’s the only language I know that can interface with both Objective-C and C++. The downside with D would be that the interoperability with Objective-C is lacking some features.