Renardo11, I'm sorry for editing this discussion, but is was already misleading. You were right, it is not the case of ”nume predicativ” here. I decided to leave in the discussion just the relevant information.
Both forms are syntactically correct (with un and without). Eu am un certificat de limba engleză can mean two different things: 1. I have a certificate in English or 2. I have one certificate in English.
In Romanian language, the definition of the ARTICLE is: ”...parte de vorbire flexibilă cu rol de instrument gramatical, care însoțește în unele limbi substantivul și arată în ce măsură obiectul denumit de acesta este cunoscut vorbitorilor.” source: https://dexonline.ro/definitie/articol (Dicționar de termeni lingvistici 1998)
The important thing here is în ce măsură obiectul denumit este cunoscut vorbitorilor which means to what extent the named object is known to/by the speakers.
So, the absence of the article means that the object is not known at all. In our sentence certificat has no article because we know nothing about that certificat. We know that it is one, because of the singular form of the object, and we know that is de limba engleză. Following this logic - ”Eu am certificat de limba engleză” can mean a lot of things, but also can mean nothing (if the person is joking, lying etc).
So the sentence Eu am certificat de limba engleză is just a general statement.
Using any of the articles here, will change the meaning of the sentence, because we will start to know something about that object.
Of course, there is a better explanation, but I could not find it by now.
Ctrl+F article usage and omission www.seelrc.org:8080/grammar/pdf/stand_alone_romanian.pdf