Eliot's Reluctance to Translate

The way words can never be translated exactly fascinates me. Switching the same message from one language to another always leaves bits of the original meaning behind (much like how some elements of music are left out in MP3 compression, for example).  Nevertheless, translation is an important tool – imagine being unable to read Homer or Dostoyevsky because you haven’t mastered Greek or Russian.

That being said, I still wonder why T.S. Eliot included several other languages in The Waste Land. What did he want to preserve in the original texts by leaving them untranslated? He begins the poem with an epigraph written in a mixture of Greek and Latin. In part I, includes multiple lines in German (lines 12, 31-35, and 42) and concludes “The Burial of the Dead” with some French that seems to break the fourth wall. Eliot’s notes cite Tristan und Isolde for some of the German lines (specifically 31-35 and 42) yet leave the lines untranslated. The French is uncited as well; it seems more of an exclamation, enclosed in quotation marks in the poem but not a reference (as far as I can find) another piece of literature.* Curiously, Eliot flips things around when it comes to referencing Dante’s Inferno – he uses English in the poem itself and Italian in his notes.  So, I suppose, my questions are these: What is Eliot so intent on preserving in each untranslated line? Why, keeping this penchant for other languages in mind, is the Inferno an exception?

*Though untranslated in the poem, a quick Google search turns up translations galore for all of these quotes. I haven’t addressed them here, most simply because I’m not sure how to at the moment. This is the second time I’ve read The Waste Land, but it’s still rather mysterious to me.