What is the difference between de corto plazo and a corto plazo (or de largo plazo and a largo plazo), meaning short-term and long-term? In what contexts can each be used?
De corto plazo refers to a property of the object it's referring to, for example you could say
'El proyecto es de corto plazo'
meaning that it is expected to be done in a short period of time, it could be something small, so it's referred as short term.
On the other hand, when you say a corto plazo, what you mean is that you (or someone else does) consider the object it's referring to as short term, for example a goal is a short term achievement for a larger objective. You could say,
'Quiero aprender un nuevo idioma. Mi objetivo a corto plazo es aprender a comunicarme de forma básica en el nuevo idioma.'
meaning that in the short term, that it what you should be doing, as opposed to in the long term. In the example, the long term objective is being able to speak another language fluently.
From my experience (Argentinian), 'a corto plazo' is mostly used, I've never heard of anyone using the phrase 'de corto plazo'.