How does the concept of poetic language appear in Russian Formalism and New Critics, and why is it of limited interest to structuralism?
Russian Formalism and New Criticism are not identical movements or philosophies (see the Q&A link below for a discussion of their differences), but they both clearly valued the local variations in language in a given literary work, particularly in poetry.
The Russian Formalist Roman Jakobson famously described literature as "organized violence committed on ordinary speech." Poetry takes everyday language and estranges it, denatures it, renders it down, recasts it in new forms... whatever metaphor you would like to use probably fits here. Poetic language is valued completely differently than the purpose-driven language of prose. Similarly, John Crowe Ransom saw each poem as possessing both "a paraphrasable core" (a general meaning of the poem that could be restated in prose) and a unique and local "texture" (a uniqueness that would be lost if restated in prose). Both critics focused mostly on poetry.
The structuralists are at the very other end of the spectrum. They do not look at small, local, unique texture in a given literary work. Rather, they look at how that small piece has meaning because of its connection to a large set of literary works. From what I've read by structuralists, there's no particular love for poetry or even much recognition for specific literary genres just as there is little interest in thet local. The tragedies and comedies of Greek drama might be analyzed as parts of one large whole, as might the creation stories from dozens of distinct and different Native American cultures.