No API endpoint for that yet.

This will not be as simple.

Onedev can't detect what programming language the codebase is, only what filetypes it detects.

It would have to return something like the percentage of filetype within the repository.

Enry is a Java library that can detect the programming language of files and repositories. It's a port of GitHub's Linguist, which is widely used for language detection in repositories.

Key features:

Supports a wide range of programming languages
Can analyze both individual files and entire directories
Uses various strategies for language detection, including filename, extension, and content analysis
Provides both accuracy and performance

It will give the same language detection, that Github provides. That is particularly good for the project, that the OP has in mind.