A rant on partial understanding

I was eating dinner in a French restaurant some years back (Pergola Des Artistes at 252 W. 46th St. in Manhattan; excellent food) and in the men's room was a poster of an ocean liner with a French inscription more or less as follows (I didn't have pen and paper with me, and may have misremembered something):

NORMANDIE
Cie. Gle. Transatlantique
60 Voyages
400.000 Milles Marins
115.000 Passagers
Au 1er Janvier 1939

Now, I already knew that "Cie." means "Company", and a bit of thought told me that "Gle" must be "Generale". "NORMANDIE" is the name of the vessel, as I already knew, and the numbers and date are obvious: sixty voyages, 400,000 nautical miles, 115,000 passengers, and the first of January, 1939. ("English is just misspelled French.")

In short, without knowing any French at all, I understand all the 10-euro words on this poster, and only one tiny, tiny detail escapes me:

Does it mean "since 1 January 1939" or "before 1 January 1939"?

Which of course is a point that is doubtless obvious to any French three-year-old, who does not know the big words at all!

I have similar problems with international auxiliary languages, where again the roots are plain but the grammatical endings are often not. Don Harlow (I think) says that if you learn that droni means "to drown", you haven't learned it (because the subject of "drown" in English can be either a semantic agent or a semantic patient); whereas if you know that it means sufokigi in akvo, you understand it. But this helps me not at all, because I can't remember if it's sufokigi or sufokiĝi, and because the English verb "suffocate" also can take an agent or a patient subject, and I know that the Esperanto suffixes -ig- and -iĝ- convert between transitive and intransitive forms, but I can't remember which is which!

Now I know perfectly well that I can look both these points up. But being vague about them interferes with on-the-fly comprehension, which is what one needs most of the time. Another fine example, from the days of King Canute, but devised by Tom Shippey (the Tolkien and Old English scholar):

Consider what happens when somebody who speaks. . . Old English. . . runs into somebody. . . who speaks good Old Norse. They can no doubt communicate with each other, but complications in both languages are going to get lost. So if the Anglo-Saxon from the South wants to say (in good Old English) "I'll sell you the horse that pulls my cart," he says: Ic selle the that hors the drægeth minne wægn.

Now the old Norseman -- if he had to say this -- would say: Ek mun selja ther hrossit er dregr vagn mine.

So, roughly speaking, they understand each other. One says wægn and the other says vagn. One says hors and drægeth; the other says hros and dregr, but broadly they are communicating. They understand the main words. What they don't understand are the grammatical parts of the sentence.

For instance, the man speaking good Old English says for one horse that hors but for two horses he says tha hors. Now the Old Norse speaker understands the word horse all right, but he's not sure if it means one or two because in Old English you say "one horse", "two horse". There is no difference between the two words for horse. The difference is conveyed in the word "the" and the old Norseman might not understand this because his word for "the" doesn't behave like that. So: are you trying to sell me one horse or are you trying to sell me two horses? If you get enough situations like that there is a strong drive towards simplifying the language.

And so it goes.

("I know one of these is my right and one is my left. Left? Right? Right? Left?")