To make an Arabic character set is not a difficult task, redefining every character with a Basic listing.

Hmm, any documents about this "Basic listing" please? I'd like to try with some characters and see.

In addition to this RtL, Arabic characters change their shapes based on their positions in words. For example, a character such as م can be at least written in 4 different shapes (isolated, initial, middle & end)!

Arabic is indeed a complicated language. Proper Arabic support on modern computer also needs to support diacritics, where characters change their shape according to their neighbors (that was apparently what you meant).
The Sinclair ZX Spectrum actually was available with an Arabic ROM as well, supporting RTL writing, but no diacritics. It's fun to at least try that out in an Emulator (Right-toLeft BASIC programs do look weird!). The ROMs are available from WoS.

Tobias

Hmm, any documents about this "Basic listing" please? I'd like to try with some characters and see.

On page 105 of the Basic manual you can read how. On the easy example made with binary numbers you can see that the 0s will be seen as paper, and the 1s as ink on every of the 9 lines of every character. Avoid using the outer bits on every line, 0 and 7, because on the TEXT80 mode they are used to represent colours and will not be seen.

You can use only one line to define a character, without DATA and READ statements:

Try on immediate mode:    SET CHARACTER 142,126,66,66,66,66,66,66,66,126          Then press ENTER.

(There are other 2 methods to define characters. With a Scape sequence written to the text channel, usually #102: ESC, K, n, r1, r2, r3, r4, r5, r6, r7, r8, r9. And the other, writing directly to the memory positions  B480h...B8FFh on segment FFh.)

Remember that the definitions will remain only until the next hard reset or the CLEAR FONT direct command being executed.

The "definable" characters are 128, from 32 to 159. If you try to define over 159 then you will be defining again on the lower numbers and if under 32 you will be defining a higher number. For example 160=>32, and 0=>128. The cursor you see on the Basic editor is defined on character 142.

Try your redefined characters executing something like this on immediate mode:

PRINT CHR$(XXX)        and then the ENTER key.

Or print all the set with:

10 FOR A=32 TO 159
20 PRINT A,CHR$(A)
30 NEXT