Not aware of the term “modelling language”? Basically, it’s a set of instructions that can be used to create the design and constructions of structures. It could be textual and graphical, both. The graphical one is easier to look at and figure out how various components of the project are related to each other.
It’s not just a drawing tool. It implements a fully-compliant UML 2 data model. You can create highly complex models with Gaphor.
See Gaphor: Open Source Graphical Modeling Tool - It's FOSS
Gaphor is a free and open source modeling application with support for various modeling languages such as UML, SysML, RAAML and C4.