I reckon I go alright in English. According to testyourvocab.com I know about 30,000 words.

Here are some of the words I didn’t know:

embonpoint
pabulum
pother
valetudinarian
cenacle
vibrissae
cantle
estivation
regnant
terpsichorean
clerisy
deracinate
fuliginous
oneiromancy
tatterdemalion
williwaw

(Pretty sure some of these are just made up.)

10,000 words in a language is generally considered fluent, so I’m comfortably fluent in English (as you’d hope).

In Spanish … not so much. I’m somewhere between 500 and 1000 words at the moment. 3000 words would have me be conversational. That would cover around 95% of general conversation and be enough to follow most conversations. That’s what I’m aiming for after my year in Argentina.

Here’s the thing. The approach to learning to be conversational is different from the approach to becoming fluent. For most people learning a language to be conversational is fine. Yet most language teaching is aimed at fluency (remember high school French?).

Just because you have mastery in your thing, it doesn’t mean that’s what you should be teaching. Maybe teaching competence would serve your clients more.