I think this must originate from Chile; all the relevant references to "profesor informante" I can find come from there. It is contrasted with "profesor patrocinante" or "guía). The latter is clearly the dissertation supervisor (director), but it's difficult to discern the exact functions of the "informante".

The ST suggests that "profesor informante" is simply a synonym of "co-director": i.e., co-supervisor. However, I don't think that is exactly the case.

The following regulations for a master's degree (in Maths at the University of Bío-Bío) indicate that the dissertation is examined by the supervisor (profesor guía, known in some places as patrocinante) and two "profesores informantes" appointed by the academic committee for the degree programme. I can't see any sign that the "profesores informantes" actually have a direct teaching role in supervising the writing of the dissertation, so I don't think they can properly be called co-supervisors. If the supervisor (guía) is external (from another university), then an internal co-supervisor (co-guía) is appointed as well (this is actually standard practice internationally):

"Artículo 7.4: Un estudiante estará en condiciones de iniciar su tesis de grado una vez que haya cumplido con los siguientes requisitos:
1. Que sea patrocinado por un Profesor Guía (académico del claustro del Programa de Magíster con menciones) aprobado por el Comité Académico del Programa. [...]
Artículo 7.5: La Tesis de Magíster concluirá, en su primera etapa con la entrega de un informe escrito, en cuatro (4) ejemplares formato carta, no empastados de la Tesis; uno para el Profesor Guía, dos para los Profesores Informantes y uno para el Comité Académico. El Profesor Guía y los Profesores Informantes deberán evaluar el Informe Escrito en un plazo no superior a un (1) mes y con un mínimo a 2 semanas.
Artículo 7.6: Los Profesores informantes serán designados por el Comité Académico del Programa, a proposición del Profesor Guía.
Artículo 7.7: La comisión que evaluará el informe escrito de la tesis estará conformada por el profesor Guía y 2 Profesores informantes designados por el Comité académico del Programa.
Artículo 7.8: En el caso en que el profesor guía sea externo, la comisión que evaluará el informe escrito de la tesis estará conformada por: El profesor Guía (académico externo del Programa de Magíster con menciones), co-guía (Profesor del claustro del Programa de Magíster con menciones) y 2 Profesores informantes designados por el Comité académico del Programa, donde uno de ellos debe pertenecer al claustro el Programa."
http://www.ubiobio.cl/miweb/web2012.php?id_pagina=3271

Sorry for the lengthy quote. I think this indicates that the "profesores informantes" are essentially examiners. "Informar", in this context, basically means "assess". So you could call them "assessors".

However, I'd like to suggest the alternative of "reviewers", partly because the following, which is on doctoral theses, suggests that the "informantes" also exercise oversight during the degree course, which means that they are appointed before final submission and are not just thesis/dissertation examiners:

"Las actividades anuales de seguimiento corresponden a la rendición anual de los resultados del desarrollo de la investigación y son de responsabilidad de los profesores informantes interno y externo, quienes deberán reportar al alumno un informe escrito una vez al año"
http://posgrado.letras.uc.cl/images/ReglamentoconDecreto2016...

So it appears that they monitor or review the candidate's progress as well as and before examining his/her dissertation.

I found the following, in English, on a joint master's programme at the University of Heidelberg in Germany and the Pontificia Católica Universidad (PUC) in Chile. This is on the Chilean "Comité de Posgrado":

"The Master’s thesis shall be submitted in due time in triplicate to the German Examination Board and in duplicate to the Chilean Comité de Postgrado [...]
The Master’s thesis shall be graded by two examiners. The first examiner shall be the supervisor of the thesis. The second examiner shall be nominated by the Comité de Postgrado in consultation with the German Examination Board. [...]
The first reviewer may return the thesis for reworking if from his/her point of view it does not meet the requirements of adequate work, but is capable of being improved."
http://www.heidelbergcenter.cl/medicalphysics/wp-content/upl...

It's not that this has any official status as a translation, but I think it fits pretty well in the light of what can be gathered about the functions of a "profesor informante".