Does learning German and reading Kant's Critique of Pure Reason in German add clarity or insight or improve the understanding; or does reading the English translation suffice?
How much more significantly does reading the German original help?
I have some points for you to consider:
1) The English translations I have read so far were trying to get the thought, but actually did miss it from time to time. That's a big problem when reading Kant, because in most cases each sentence has its weight and if you do not get one right, you could have trouble to follow later parts of the text.
2) Sentence structure. Kant himself wrote the sentences in Latin grammar taking the overall structure. That means if you want to fully and correctly understand the German text you will have to learn both German language and basic Latin grammar. To make it even more complicated, the subsets of the sentences are in (relatively old) German grammar and vocabulary. This is what makes it so hard to translate and/or understand it.
3) In German the best introduction to understand the conceptual aims and backgrounds of CPR is from Eckart Förster, Die 25 Jahre der Philosophie. He reconstructs the history of the ideas behind the CPR and why it is construed as it is and has to include what it does. An English translation is available published by Harvard University Press. In addition, it is described how Kant set the ball rolling, culminating in German Idealism in general and Hegels Phenomenology of Mind in particular. So all major authors of this movement are introduced and better understood by reading this book. And as far as I can take the reviews seriously, the main benefits have been preserved.
Well, even some of the best contempory Kant-experts who did learn German to read him (i.e. Allison and presumably Wood) have major problems understanding the full meaning. So it can help, but it does not have to. It depends on the reader. And this is something independent from translations or native language.
Reading Kant, e.g., Critique of Pure Reason (CPR), is a difficult task even for Germans (I am a German native speaker). Often Kant makes very long sentences. Before understanding the text one has to search first in the sentence, to which word a certain pronoun refers to. Secondly, Kants uses words which are no longer in use in German.
Hence, even after you have learned German you will not be able to decipher each sentence from the original. In any case, I doubt whether a non-native speaker can obtain the capability necessary for a verbatim textual exegesis of CPR on the research level.
I propose to rely on the job which a translator has already done for you and to read Kant in your native language. E.g., I consider the translation of Max Mueller a good translation of CPR. It is the base for the translation which appeared in the Penguin books series.
My suggestion is to start with the Preface to the Second Edition of CPR.
Why not both?
You can read in your native language and consult a German text in key passages or even for random "flavor." Some schools teach quick, condensed grammar courses in German, without vocabulary or pronunciation, so you can work with the original using a dictionary. This let you observe key terms in their natural habitat.
Many German terms (Auflklarung, Dasein)have a beauty and suggestiveness that is worth attending to. Many nouns click together and come apart like Legos, and it is helpful to play with the parts. And etymology is half of philosophy.
Of course, language is endless, lifetimes are limited.
No matter how you slice it, Kant was badly in need of a good editor, and in many respects, that's part of what a good English translation of the CPR will do. The Guyer/Wood translation is generally considered the best English translation going, and as other users have pointed out, Kant's sentence structure in the original German is confused and frightful to the point of vitiating its usefulness for any but the serious Kant scholar. I am not one such, but my work requires that I keep a hand in on Kant. For my part, I know enough German to get by, and I do find it helpful to keep a German copy of the text on hand to check points of vocabulary (because meanings can be lost in translation), but I generally find the English translation to be far clearer in most respects. I'm given to understand that many native German speakers, particularly when encountering Kant for the first time, use the German text and the English translation in the same way (though it's possible I'm misinformed on this point).
I do not recommend learning German for the purpose of reading CPR. I read Meiklejohn's English translation; I found even the English text so complex that I needed to add extensive annotation and notes to make any sense of it. I found the Meiklejohn work at Project Gutenberg. I first started reading Mueller's translation in the expectation that a native German speaker would be able to provide a better sense in English of what Kant was getting at, but the Meiklejohn translation worked better for me. For help I followed Gardner, Kant and the Critique of Pure Reason (Routledge).
Just read English version, and do read this companion book
Patton's "Kants Metaphysic Of Experience" https://archive.org/details/kantsmetaphysico033328mbp
Excellent book that gave me so much clarity behind Kant's obscure language.
Reading Kant on German would not add any value nor any original extra understanding that people didn't figured out before you. You will struggle figuring out what he meat to say, against what he said, and at the end you will not realize anything that somebody else didn't already figured out.
My take is that Kant's genius lies in his view that our mind is a systematic machine for spitting out sentences. He was the first to formulate such idea and get into the nitty gritty of all needed parts of that flow.
Understanding his message is not reserved to German version only. The point is translatable, even if some sentences don't translate the meaning perfectly.
NB: Listen to this episode of the RadioLab Podcast, it get's you into the problem of translation really nicely