Password only works with English letters? What does that mean?

This means “Plain ASCII”. So what the Amis had standardized in the 1950s.

“Plain ASCII” are 7-bit, so without “Advanced Codepage” for country-specific characters.

Here is a table of “Plain ASCII” characters

         2 3 4 5 6 7       30 40 50 60 70 80 90 100 110 120
       -------------      ---------------------------------
      0:   0 @ P ` p     0:    (  2  <  F  P  Z  d   n   x
      1: ! 1 A Q a q     1:    )  3  =  G  Q  [  e   o   y
      2: " 2 B R b r     2:    *  4  >  H  R  \  f   p   z
      3: # 3 C S c s     3: !  +  5  ?  I  S  ]  g   q   {
      4: $ 4 D T d t     4: "  ,  6  @  J  T  ^  h   r   |
      5: % 5 E U e u     5: #  -  7  A  K  U  _  i   s   }
      6: & 6 F V f v     6: $  .  8  B  L  V  `  j   t   ~
      7: ' 7 G W g w     7: %  /  9  C  M  W  a  k   u  DEL
      8: ( 8 H X h x     8: &  0  :  D  N  X  b  l   v
      9: ) 9 I Y i y     9: '  1  ;  E  O  Y  c  m   w
      A: * : J Z j z
      B: + ; K [ k {
      C: , < L \ l |
      D: - = M ] m }
      E: . > N ^ n ~
      F: / ? O _ o DEL

So you can use all these characters, except of course the “DEL” character, which deletes the previous character when entering and does not become part of the password.