Why does FIDE recommends using German numbers and piece names for blind and visually impaired? While reading FIDE version of the rules, I stumbled upon Appendix E: Rules for play with Blind and Visually Handicapped.
There, in E2.1:

The moves shall be announced clearly, repeated by the opponent and executed on his chessboard. When promoting a pawn, the player must announce which piece is chosen. To make the announcement as clear as possible, the use of the following names is suggested instead of the corresponding letters, algebraic
A - Anna, B - Bella, C - Cesar, D - David, E - Eva, F - Felix, G - Gustav, H - Hector
Ranks from white to black shall receive the German numbers:
1 - eins, 2 - zwei, 3 - drei, 4 - vier, 5 - fuenf, 6 - sechs, 7 - sieben, 8 - acht
Castling is announced “Lange Rochade” (German for long castling) and “Kurze Rochade” (German for short castling).
The pieces bear the names: Koenig, Dame, Turm, Laeufer, Springer, Bauer.

So, FIDE recommends using German for announcing moves when tournaments with blind/visually handicapped participants take place.
A quick search did not allow me to answer the question of why is it like that and why German (not, say, French) was chosen. I also wonder, since when did those recommendations are present in the rulebook.
Not sure but maybe it is because International Braille Chess Association headquarters are in Germany, also as far as I know ( My knowledge is on the basis of Chess Base India's videos ) players are free to announce their moves in any language as long as both of them agree, German names are used only if the players are not able to reach any mutual agreement.