Many programming languages share generic and even fairly universal features. For example, if you compared Java, VB6, .NET, PHP, Python, then you would find common functions such as control structures, numeric and string manipulation, etc.

What has been done to define these features at a meta-language (or language-agnostic) level?

UML offers a descriptive reference of software in every aspect, but the real-world focus seems to be data processes. Is UML relevant?

I'm not asking "Why we don't have a single language that replaces the current plethora." We need many different tools (at least in this eon).

I'm not asking that all languages fit a template -- assembly vs. compiled languages are different enough to make that unfeasible (and some folks call HTML a language, though I wouldn't). Any attempt would start with a properly narrow scope. In line with this, I wouldn't expect the model to cover even a small selection with full validity.

I would expect however that such a model could be used to transpose from one language to another (with limited goals -- think jist translation).