As a former foreign language major and as a current foreign language teacher (ESL/Spanish), I often deal with this topic. Simply put, neither my colleagues nor I have come to a satisfactory answer.
In the military we had a standardized test called the DLPT, so we could list our DLPT scores (for example: Reading 2+, Listening 3). Europe, if I’m not mistaken, is moving towards adapting a skills-based rubric that places one on a scale from A1 to C2, but it hasn’t made its way to our side of the ocean yet
So what to do? Terms like fluent, proficient, and conversational are ill-defined, as are terms like basic, intermediate, and advanced. However, they are all that we have. Luckily, the latter set of adjectives can be linked to commonly used college classes descriptions. French 101 and 102 tend to be Basic (or Elementary), 201 and 202 are Intermediate, 301 and 302 are Advanced, etc. As an example, the highest level French classes that I took were the lit/civ courses, as well as a semester abroad and a 50-page research paper. So I put French: advanced, but rusty. Also, I took 4 semesters of German, so I put German: intermediate. I also studied some Slavic, Semitic, and Mon-Khmer languages, but only for one semester. To avoid appearing as either a braggart or directionless dilettante, I put none of them on my resume.
However, not everyone has obtained skills from college. In this case, I would suggest a guestimation. Can you conjugate the present, present progressive, past, future, and conditional tenses? Do you have the vocabulary to talk about food, sports, family, school, body parts, etc.? Then that’s equivalent to about two semesters (Beginner). Can you use the subjunctive mood and tell your doctor what hurts and what happened to you last night after you left the bar? Can you read the paper and get 90% of what’s going on? Then you’re looking at Intermediate. Can you distinguish between the preterite, imperfect, present perfect, and past perfect as well as conjugate them all without blinking? Can you read an Op-Ed about Bolivia’s constitutional crisis? Can you understand little kids and less educated speakers, as opposed to just CNN-style news broadcasts? Then put Advanced.
IMHO: Conversational is too vague to mean anything. Proficient is better, but still open to interpretation. Fluent, for me, is a strong word. If you can not only read said Op-Ed about constitutional crisis, but write your own publishable response to it, then go for it. Otherwise, play it safe. We gringos tend to overestimate our capabilities.