bassoonery: - the instructions for the second grid (the one without the given T) is implied by "the corresponding cell in the other grid will be occupied by a different letter". You have two solutions which can be differentiated by the last digit of 8d; the one where 8d ends in T goes left, etc. I realize it does not say this explicitly, but it's a reasonable inference.
Cruncher - Look at 12d; that makes it clear that when a expression is to be squared it will be enclosed in parentheses. so (BXDXBXD) would have been clued as (BD)^2.
I'm sure this would get the prize for the most poorly worded set of instructions yet.
The instructions are flawed because you cannot really deduce all of the answers our dummkopf solver transcribed into German. You can deduce answer that MUST have been entered that way, but there are answers for which you cannot know which language was used. (Suppose an answer were 777777 - that becomes SSSSSS either way and you cannot tell if this was an English or German entry). I realize this is a nit; but such poorly written instructions are begging to be faulted.
There is a logical path which is somewhat less computationally expensive than brute force; e.g. 24a and 15d constrain B and D somewhat. If you continue in that fashion you quickly(?) get a set of 5 or 6 letters that are constrained to one solution or the other...but it was still a slog.
I think this is a more coherent set of instructions:
The letters in the clues stand for the first ten primes (2, 3, 5, 7, 11, 13, 17, 19, 23 and 29) and lead to numbers that are all different, none beginning with a zero. The notation A^2 is used to mean A squared, and A^3 for A cubed.
The intent was that each digit was to be entered in the grid as the first letter of its lexical form (1 = One, 2 = Two, 3 = Three, 4 = Four, 5 = Five, 6 = Six, 7 = Seven, 8 = Eight, 9 = Nine, 0 = Zero) so, for example, 3456 would be entered as TFFS. HOWEVER, this puzzle was adapted from a version in a German newspaper* by an enthusiastic local solver who inadvertently transcribed some clue answers using their German format (1 = Eins, 2 = Zwei, 3 = Drei, 4 = Vier, 5 = Fünf, 6 = Sechs, 7 = Sieben, 8 = Acht, 9 = Neun, 0 = Null), which would make 3456 DVFS. Regrettably, this resulted in two valid solutions. Solvers should enter both of these in the pair of provided grids; the given letter will determine which solution goes in which grid. Solvers should identify those answers in each grid that were entered in German and consider their English counterparts in clue order. For one of the grids, this will result in two words of equal length that are to be entered below the grids, one on each side of the colon; this will relate to a claim made above.