From MusicBrainz Wiki

About turning instruments into entities.
The current implementation of instruments has some limitations. Some instruments have different names, some different instruments have the same name. Adding to this is the problem where a user using a translated version of the site will have to search in a localized instrument list, but chances are he or she is using a different-language source (usually English), resulting in confused users at best, and faulty edits at worst.
Turning instruments into entities would be able to solve those problems in the following ways:
1. Instruments with multiple names can have multiple aliases for a given locale. A user searching for one name will always find the correct instrument, as long as the alias is present.
2. Different instruments with the same name can be differentiated with a disambiguation comment.
3. Translated instrument names will be present as localized aliases. An editor looking for a Klavecimbel or Cembalo will find the instrument called Harpsichord in English.

There are a couple of details that have to be worked out:
1. Having to add an alias for every instrument is a lot of work for a translator, and a lot of languages have already had the instruments translated. Ideally, the localized aliases should be seeded from Transifex.
2. Right now instruments are ordered in a tree. That can be emulated with relationships between instruments and their parents. The exact wording of the link phrases will have to be worked out.
Example: [instrument 1] is a child instrument of [instrument 2] and [instrument 2] is a parent instrument of [instrument 1]
3. Instrument descriptions are now part of the site. This could be changed to annotations and/or Wikipedia links. The beta site already shows the first paragraph of Wikipages for other entities like artists. These pages are localized in the language the site is viewed in, as long as a localized Wikipedia page is available as well. Annotations are not localized though, so using Wikipedia could be necessary. For MusicBrainz, a stub on Wikipedia with rudimentary information about the instrument is enough, but the editors at Wikipedia may disagree. They may also start whining about notability again. Is that still much of an issue there?
4. Images of the instruments would be nice, perhaps they can be sourced from WikiMedia. But this is a feature that could be added at a later date, and not really important now.

Original Jira ticket
Mailing list discussion