I found a glitch that allows for the Word tab of non word tab languages to be visible.
I was doing some French, and just casually checked my Words tab, like I do after I finish a few skills. The word count was at 2361, and I thought that was exorbitantly high. I noticed that it was my Swedish word list. There were some glitches, but I could do flashcards and it even displayed word strength. The flashcards even came with the little circular Swedish flag. From this, I am wondering, why can't they just switch the words tab on for other languages if it already works?
They might be testing it. A week or two ago, some users of the English for Czech course reported that they had the Words tab available for them, but most still have a 404 page instead.
It's a fairly well-known issue that results from the site not being designed to show 2 different courses at the same time, even in different tabs. Here's an old thread that documents a procedure you can use to recreate it.
Like you mentioned, the words tab is mostly usable this way. You might see some "words" that aren't really words in the list, but the flash cards should be fairly solid. You might also have issues earning XP using the site in this way, since it likely will be assigned to the other course you have open (French).
To answer your ending question, generally sites don't like to put "in progress" material live, as it doesn't give a good impression to those who aren't aware it's "in progress".