The English OF account for only about 5% of all OF Masses. How can that be
ordinarily celebrated?

Ordinary can be a universal and a relative term.
In the US, as in all other countries, the OF is the ordinary form of the Mass. Also, I would expect to find a Mass in the US being done in the most common language in the US, which is english. If I see “Mass” in a US bulletin without qualifications, I assume it means english OF because that’s just how you ordinarily speak with people. If it were the 1940s and saw “Mass” I would expect to see the Tridentine Mass in latin (no idea what year Missal would be used at that point).
If I were in Spain, it’s the same train of thought. The OF is ordinary, and Spanish is expected because it’s the language of the country. If I saw a listing of Masses in Spain, unless otherwise noted I would assume they are spanish OF Masses.
I’m sorry if you’re peeved about it, but people speak informally, not legally. When you speak informally, you leave out unneeded descriptors like “english OF” when you say Mass because statistically speaking (if you’re in the US) there probably upwards of 90% likelihood (probably near 97-98% is my guesstimation) that it’s an english OF Mass.
This also applies to our Easter Catholic brethren in North America. We don’t have to specify Roman Catholic because if we say Catholic people normally think of Roman Catholics, but others may have to specify Ukrainian Catholic because it’s less common. If we were in Ukraine however (or anywhere else with a sizeable Eastern Catholic population), I’d have to say Roman Catholic.
You can see it’s the same thing with forms. If the EF ever got to say 10-20% by proportion of total Masses in the US (unlikely to happen anytime soon), then you’d probably have to start specifying to avoid confusion. Right now we don’t have to; when you say Mass people think “english OF Mass”.