As defined by UNESCO (2021) “Open Educational Resources (OER) are teaching, learning and research materials in any medium – digital or otherwise – that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with no or limited restrictions.” OERs are free of copyright content and easy to modify; moreover, they can usually be found in multiple trustworthy locations and are clearly licensed (e.g., through Creative Commons).
There is a huge variety of digital resources available on the Internet that can be reused for teaching and learning, e.g., educational materials (textbooks, syllabi, course activities, readings, etc.), Massive Open Online Courses (MOOCs), among others. Besides being free of charge they can also be found in multilingual format, since they are produced around the world. You can find resources for your teaching in content libraries and collections, using key terms and/or filtering by publication date, language, educational level, etc.