Arabic is right to left and joins letters
Arabic letters connect to each other, and the shape of a letter changes by position. That means letter-spacing breaks the script, so itistitle never adds it to Arabic text. Lines are set right to left and aligned correctly.
Fonts to try for Arabic
Cairo is clean and modern; Tajawal is light and contemporary; Noto Kufi Arabic is geometric and strong, good for bold covers. Pick one from the “Arabic” group of the font list.
Malayalam
Malayalam has rounded forms and stacked conjuncts that need a font with full coverage. Noto Sans Malayalam is included and works at normal and heavy weights. Keep titles short, as Malayalam words tend to be longer than English ones.
Mixing scripts
You can write the title in Arabic or Malayalam and the subtitle in English (or the reverse). Each line is analysed on its own and set in the right direction. If you use an English-only font for a title in another script, itistitle falls back to a font that has the letters, so you never see empty boxes.
Tips
Use slightly more line height than for English, avoid italic styles (they don’t exist in Arabic), and check your text with a native reader before publishing.