Which dictionary? Who defines it? Who decides when Kleenex or App are to be included? Are words ever removed from that dictionary? What about foreign language dictionaries? What about foreign words transliterations to a different alphabet? What about made-up words that sound like an existing word with different spelling? Acronyms?
Excellent question, it would be wonderful to have a world dictionary of words in usage with standards on how to add or remove words in a democratic way. For now an example would be to pick the 5 most used dictionaries in that language. I agree that it should include all languages, perhaps a database would need to be created of forbidden words that could be easily searched. I certainly haven't worked out all the details, but if it were implemented then these problems could be solved.
These questions are are pretty well answered by trademark law. Most of it boils down to a judge's decision, who, by the way, can decide that a hereforeto novel term has become generic (much as Xerox, Kleenex, and Photoshop are in danger of becoming), hence why you see ridiculous efforts by companies with household names to prevent verbification of their names.
I know, I was challenging the idea that it should not be allowed to trademark words in “the dictionary”. It’s a judge decision, there is no formal list of disallowed words, and my point is that there can’t be one, in the real world.
