The “DLL” stands for Dynamic Link Library. This particular DLL’s job is to store language-specific data (menus, dialog box text, button labels) for a parent program. The seemingly random name s7yomlr is often an used by developers to avoid naming conflicts or, in rare cases, to hide the file’s purpose.
If the file is truly missing but a language-specific version exists in C:\Program Files\siemens\step7\s7bin , some users resolve this by copying S7YOMLRB.DLL (the English version) and renaming the copy to S7YOMLR.DLL Registry Cleanup: cannot find language resource file s7yomlr.dll