Poor flexibility in acceptable responses
I tried Duolinguo out this morning but will already be quitting. My Spanish is much, much better than my German, yet your initial test put me at 17% Spanish and 37% German. I have had trouble advancing in testing out of things because your acceptable answers are extremely inflexible and demand extremely direct one-to-one translations that are either awkward in the other language or just as right as the one your system accepts as correct. The issues are so basic that at times, your system does not account for the equal acceptability of translating "you" as either "du/Sie" or "tu/Ud." even when there is no context to indicate whether the formal or the informal form is indicated. This has potential, but meanwhile, you need to work on your software and hire more language consultants.
Don't worry about the percentage, I think I got 30-something percent for German (from English) in the placement test and I am German. :) And I got a much higher percentage for English from German five minutes earlier. Now I've finished the German tree, I'm at 64%. My French ist at 57% and I've got half of the tree still left. At the beginning you seem to start with a very low percentage, then rise quickly, then - from about 50% upwards - Duo gets really reluctant to hand out additional percentage points. The whole thing doesn't have much to do with fluency.
Actually Duo does accept both "du" and "Sie" (or "jij", "je", "u" and "jullie" in Dutch). If it doesn't, you should report the sentence and it ought to be corrected.
As for the awkward translations, well, yes, they sometimes are. Sometimes I sit there wondering what to type, I know what the sentence means, but if I use a nice English phrasing, it's probably too far from the original to count, and sometimes if I choose a very literal (and awkward) translation, Duo considers it too literal... but after all, this isn't a competition or game where points/mistakes count for anything.
I don't mind having to type in awkward translations. To me, it's not about writing elegant translations into English, but about proving I understand what I'm supposed to translate. Sometimes you do get the odd bit that's really awkward, but taken all in all, I think Duo is a great help for learning languages. It's neither perfect nor complete (you don't really learn a lot of different words, all in all), but it's good for basics, for practising, ...
Maybe it's not what you need, but maybe you'd like to see if you can get used to the way Duo works and find out how it can help you. This isn't my usual favourite method of learning either, but I feel it really helps to hammer home some words and grammar.
Not sure if you're aware that the courses (and translations) are put together by volunteers. No language consultants are hired. And I have seen some REALLY bad translations proposed on the forum, just because the word in question CAN mean something different than what DUO wants. Really, this type of teaching isn't intended for elegant or smooth translations, just adequate translations. Nor should it allow EVERY variation of meaning of a word - sometimes that actually leads to nonsensical (but grammatically correct) sentences. And there are those who argue that DUO can sometimes be TOO flexible. Report those translations that you're using that aren't accepted. If the team volunteering it's time decides that your variation is acceptable, they will eventually add them. It takes a while, sometimes.