The phrase "It speaks by itself" simply doesn't exist in English, except in the most obscure and bizarre circumstances. Perhaps if you had a toy that spoke, and you wished to clarify that it wasn't speaking with other toys. That's when "It speaks by itself" would be useful. The phrase, "It speaks for itself," however, is a common idiom in English.
"It speaks BY itself" actually does make sense, within certain contexts. "Es spricht von allein" means "it speaks of itself," and more approximately "it speaks BY itself" (or "it is speaking by itself.")
We might imagine a doll that turns its head toward you and says "I'm Krusty the Clown, and I don't like you." And you realize, in a cold sweat, that no one has pulled the doll's pull-string. So you say "it's speaking by itself!" (Es spricht von allein!)
Read what mick1203 said right above your comment for further explanation :)
"It speaks on its own" or "it is speaking on its own" is fine as well; and so is "it's talking by itself. I, personally, would say "it's talking by itself" (within the context of the doll). I see no stretch here, and I been talkin' English good since forever. (maybe it's a regional difference? I'm from the West Coast of 'Merica).
The question isn't whether or not "it speaks by itself" is OK under certain contexts—the question is whether or not the German sentence is such a context. If the German sentence is trying to express the idea that "it is self evident," then the preferable way to express that in English is "it speaks for itself."
I think we've figured out the context already.
"It speaks for itself" = "Es für sich sprechen."
"Nein, die Beweise sprechen für sich." ("No, the evidence speaks for itself.")
We, on the other hand, are talking about someone or something doing something without being prompted by something else. We are not talking about matters of self-evidence.
It is speaking by itself: it is speaking but there are no listeners. It is speaking for itself: either it is speaking in its own advocacy (unlikely today, future? Who knows...) or it is speaking from its own volition (maybe just apparently). It is speaking to itself: hmm can an it be its own listener?
"It speaks for itself" and "It speaks by itself" are translated with "Es spricht für sich selbst" (DeepL) and the meaning is, a process, a desicion, an expression must not be explained by additional words. I do not know, what the meaning of the given German sentence "Es spricht allein" shall be. Each person, who is speaking, speaks alone without external help. I have never heard that sentence in my entire life in Germany?! ))-:
All of the comments here are about speaking. I am amazed that none of the comments pick up on the point that actually this sentence is not about speaking at all. I think that is where a lot of confusion has arisen. So something, an inanimate object and not a person, does not 'speak' in any way at all either by itself or with any help.
Guys, this sentence is what is known as a synonym. Forget anything to do with speaking or talking, so trying to get the speaking right is totally irrelevant. To explain, If something speaks for itself, it is clear and needs no further explanation, for example, 'The school's excellent record speaks for itself.' Just an example hence the 'it' in the translation as there is no person or speaking involved. On that basis we can just go with the German translation and not get hung up about any speaking - which never takes place anyway. I hope this helps.
No matter what object or person is speaking "von allein" with "by oneself or for oneself", the sentence is meaningless in English and in German. The only meaningful sentence I can think of in this context is: "Es spricht für sich selbst = It speaks for itself" and that has the meaning, it does not need to be explained, the facts are self-explanatory.
Then would you explain why "es" is chosen instead of "er" or "sie" which would make the sentence more sensible ? And isn't "er spricht für sich selbst" more meaningful an natural ? Why choosing a number of sentences which have a potential of dispute among even the natives ? Yes I accept the sentence is not wrong, and it may seem totally normal to you as a native; however, another native does not find it as usual as you think. Because this is a valuable learning platform, the contributors should do their best for refraining from confusing the learners. If you are closely looking at the criticisms in the discussions you should see a huge number of complaints just about this issue. If you are among the contributors of this session (thanks a lot for your efforts of course) please take these criticisms seriously especially when they come from a learner having considerable streaks !
"Es" is chosen because they're talking about an "it" rather than a "he" or a "she." One could be talking about a toy that usually needs to have a string pulled or a button pressed to make it talk, but it talks by itself because the button is jammed or the string is broken (which is not an unrealistic scenario). It could also be the case that a child who is beginning to speak on its own is being referred to derisively as "it," rather than as "he" or "she." "It's speaking by itself."
It doesn't matter if the sentence or the context is obscure. There is nothing grammatically or contextually wrong with it.
Furthermore, your streak is irrelevant. Some of us have jobs, and other responsibilities and other hobbies. It doesn't make us less diligent learners. However, if you want to go that route, consider this: some of us are several levels higher than you in German. Does that mean we take it more seriously than you? Not necessarily.
First, streak is not irrelevant, if it was DL would not emphasize it. Second, it definitely shows one's enthusiasm and interest in DL, although the opposite is may not be true as you mentioned. I love DL, I will continue learning through DL, however I would be happier if the criticisms were welcomed (especially those are widely shared by the learners in the forums) other than the concrete faults.
Alrighty then. I'm not sure why you think your streak makes your comments more worthy and valuable than others', but anyway... Here's some advice from someone 4 levels higher than you in German, and from a native speaker of English: Comments and criticisms are always welcome, except when one continues to beat dead horses after something has already been clarified (I'm not sure why you think there is still clarification needed. See other comments between me and others). There is a difference between constructive criticism and combative criticism, and I think you are beginning to stray into the latter territory.
If you still think, after all this discussion, that there is something wrong with the sentence "Es spricht von allein," and its translation, then by all means, report it to Duolingo. I don't think there is much else that can be done here.