That should be "kompleta", not "plena". "Plena" would be for a container that is full of something, like a full bowl of soup or a full glass of water or a full tank of gas.
"Plena esperantisto" would mean an Esperantist who has eaten a very large meal and is quite full with food.
I'm pretty sure the word for that (being full from food) is "sata". "Plena" can be translated into "full" but it can also translate as "complete", "unabridged". Take the "Plena Manlibro de Esperanta Gramatiko" for example.
Since "plena" appears to convey the idea of "in totality/without lack" I elected to use it. Some adjectives also express different shades of meaning when applied to inanimate objects vs. living beings. I'm not 100% if that's the case with "plena", but I wouldn't rule it out!
Funny ! festi is festa in Swedish, with 'a' at the end being the infinitiv marker, too (as the 'i'). So far in this course I only saw words 'imported' from French, Spanish, Italian, German, Lati, and English. But afaik none of them have such a close match this time :-)
just mina du cents...