Approaching this from an English-speaker's viewpoint, when I say that
I understand English, I don't mean to say that I understand ANYTHING
which is discussed in English. Thus, most English speakers don't
understand discussions of quantum mechanics.
Many people who are fluent in a language have little technical
knowledge of the grammatical principles. On the other hand, they
understand varying amounts of what is said or written.
Let's take 'Jabberwocky' as an example of where understanding a
language is clearly differentiated from lack of understanding.
"`Twas brillig, and the slithy toves
Did gyre and gimble in the wabe:
All mimsy were the borogoves,
And the mome raths outgrabe."
English speakers understand that the action takes place in the past,
and that some things referred to as slithy toves gyred and gimbled in
some place/time/manner/fashion.
We recognize nonsense verbs, nouns, and adjectives.
There have been attempts to 'translate' Jabberwocky into other
languages. Someone unfamiliar with the target language would be unable
to 'understand' any of the poem, nor recognize which words are
nonsense and which are actual parts of the language.
We can understand a language when we read it, but not when we hear it,
or vice versa.
We can understand a language without being able to speak it.
When my math teacher in secondary school required us to use Spanish to
express our need to sharpen our pencils, I understood that 'Puedo
sacarle punto al lapis' meant 'May I sharpen my pencil', but I
understood little beyond that, not even being taught that 'Puedo'
meant 'I can' (or 'Can I').
As far as I can tell, there is a certain type of neurological activity
when a person understands what is being read or heard, but does not
occur otherwise. This understanding is at two levels. The first
level is a general comprehension of the overall language/symbol system
being used. The second level is a more specific understanding of the
topic of the communication. The second level is what is apparently
addressed by Lojban. The first level is apparently unaddressed.
When I watched 'The Passion' I understood about a quarter of the
Aramaic, as I can read Syriac Aramaic with a little fluency, and have
a little familiarity with Biblical and Rabbinic Aramaic. As the topics
were simple, I easily understood them, in a way that I wouldn't had
the actors been discussing obscure technical points of Rabbinic law
('prosbul' for instance).
In the 1980s I listened to a talk by Mar Babai Soro in Assyrian
Aramaic, of which I understood perhaps 1 word in ten, if that.
However, I was able to follow well enough to understand (2nd sense)
something of what he was talking about. On the other hand, a
discussion with a friend about string theory was barely comprehensible
in the 2nd sense while fully comprehensible in the 1st sense (all the
terms were common English).
It appears we need a term to handle the first sense of
understanding--do I understand the words. The same term would be used
for understanding someone's handwriting, or someone's accent.