QuickLook is not working with mac OS High Sierra 10.13.3
Assuming your .bib file is like my simple example from https://www.latex-tutorial.com/tutorials/bibtex/#bib-file I'd say you rather want a general plain text QuickLook-plugin (aka qlgenerator), or a specific .bib one.

There might be others but I don't mind suggesting QLStephen that has been around and working fine for a long time and nicely previewed both my .bib example file and a .strings one i found; not trying to install the QL-plugin you linked; though, to verify how it works, I did download the linked plugin's source and saw its package Contents/Info.plist (showable by context menu click in Finder) having CFBundleDocumentTypes UTI com.apple.xcode.strings-text whereas QLStephen uses an extremely general fallback UTI of public.data to preview anything not picked up by any other plugin.

One could try to just modify the linked .qlgenerator's Contents/Info.plist to include something about .bib extension or UTI but that would require extending the scope to referencing 
developer documentation.