What is the English equivalent to the Japanese word 学者バカ, "Scholar’s fool"?

For a “person who is an excellent expert on one particular thing but ignorant of everything other than that in two or three words”, you might go with idiot savant, which gets the meaning across but is somewhat extreme.

Especially in an academic context, a gakushabaka might be described as an absent-minded professor.

From Wikipedia:

The absent-minded professor is a stock character of popular fiction, usually portrayed as a talented academic whose focus on academic matters leads them to ignore or forget their surroundings.

A very simple street-level equivalent for the Japanese is "book-smart".

As site participant extraordinaire Robusto points out in a comment beneath this answer, the entry for 学者バカ at jisho.org offers this definition for the Japanese word:

person who is book smart but lacking in common sense; person who is book smart but street dumb

The expression "too smart for his own good" gets close, but I'm not sure it's exactly what you want. It usually has a more restricted context (a single situation) than you seem to be looking for.

The word "specialist" explicitly means someone who is good at a particular, narrow skill. In spoken English (and to some extent in writing), the inflection can also carry the unspoken faint praise: they're skilled at one thing and not skilled at others.

The words "hubris" and "arrogance" are generalized forms of believing yourself more capable than you actually are, though they certainly carry the sense you seem to be looking for. Perhaps "professional's hubris" or "professional's arrogance" could be what you're looking for.