Gdtyjr

I use TeXWorks with XeLaTex. I want to load multiple fonts in my document. When a character doesn't exist in the main font, XeTeX should search the other fonts.

Non-working example:

documentclass[a4paper,10pt]article
usepackagefontspec
setmainfontArial
newfontfamilykoreanfontkorean.ttf
newfontfamilytradchinesefonttrad-chinese.ttf
newfontfamilysimpchinesefontsimp-chinese.ttf
newfontfamilyoldchinesefontold-chinese.ttf
newfontfamilygreekfontgreek.ttf
newfontfamilyarabfontarab.ttf
begindocument
Holá hello 们 們 안녕 𦮙
enddocument

That should produce something like this mock-up from MS Word:

Currently I get:

I can't mark up each part of the text because this needs to be dynamic. So as in Word, I nee LaTeX to recognize which font contains the character.

The literal answer to the question you asked is that iffontchar checks whether a font contains the specified glyph, and can be used to implement a fallback. However, I think that is an XY problem.

You can almost do this with ucharclasses. For this MCVE, I used the Noto Sans font family, except for one rare ideograph that I took from Babelstone Han.

usepackagefontspec
usepackage[Latin, Arabic, CJK, Greek, Korean,
 CJKUnifiedIdeographsExtensionB
 ]ucharclasses

defaultfontfeaturesScale = MatchLowercase, Ligatures = TeX
setmainfontNoto Sans[Scale = 1.0]
setsansfontNoto Sans

newfontfamilykoreanfontNoto Sans CJK KR[
 Language=Korean, Script=CJK]
newfontfamilytradchinesefontNoto Sans CJK TC[
 % CJKShape = Traditional,
 Language=Chinese Traditional, Script = CJK]
newfontfamilysimpchinesefontNoto Sans CJK SC[
 % CJKShape = Simplified,
 Language=Chinese Simplified, Script = CJK]
newfontfamilyoldchinesefontBabelStone Han[
 Script=CJK]
newfontfamilygreekfontNoto Sans[
 % Language = Greek,
 Script = Greek]
% WARNING: RTL scripts require polyglossia or babel to work correctly!
newfontfamilyarabfontNoto Sans Arabic[
 Script = Arabic]

setTransitionsForArabicsarabfont
setTransitionsForChinesesimpchinesefont
setTransitionsForKoreankoreanfont
setTransitionsForGreekgreekfont
setTransitionToCJKUnifiedIdeographsExtensionBoldchinesefont % For U+26B99

begindocument
Holá hello 们 們 안녕 𦮙
enddocument

It doesn’t quite work out of the box for all those languages you requested. First, Simplified Chinese, Traditional Chinese and Korean share many of the same Unicode codepoints, so without further markup, you cannot tell how to render them. Second, it has some problems with right-to-left scripts such as Arabic.

If you wanted to write non-trivial amounts of all those languages in the same document, you would use a package such as Babel and write things like foreignlanguagekorean韓國語 and DeclareRobustCommandoldchinese[1]oldchinesefont #1.

You could declare individual glyphs with newunicodechar, as in:

usepackagefontspec, newunicodechar
newfontfamilyoldchinesefontBabelStone Han[
 Scale = MatchLowercase,
 Script=CJK]

newunicodechar𦮙oldchinesefont 𦮙

This does not play well with ucharclasses, however.