the desired Arabic font is Droid Arabic Naskh source here. Problem is that when I compile the document using xetex I get funny boxes instead of non-Arabic charset as in the MWE below:
MWE
\documentclass[11pt]{scrbook}
\usepackage{fontspec}
\usepackage{polyglossia}
\setmainlanguage[numerals=mashriq,abjadjimnotail=true]{arabic}
\newfontfamily\arabicfont[Script=Arabic,Scale=1.2,Ligatures=TeX]{Droid Arabic Naskh}
\begin{document}
هذا نص بالعربية وفيه بعض الرموز الاخرى مثلا (-، * ، + ، )
\end{document}
Output
Question
Is there a way to provide a fallback font for Droid Arabic Naskh font to typeset all non-Arabic charsets and what fallback font would be more suitable than any other for this job?
Update
\documentclass[11pt]{scrbook}
\usepackage{fontspec}
\usepackage{polyglossia}
\setmainlanguage[numerals=mashriq,abjadjimnotail=true]{arabic}
\setotherlanguages{english}
\newfontfamily\arabicfont[Script=Arabic,Scale=1.2,Ligatures=TeX]{Droid Arabic Naskh}
\newfontfamily\englishfont{Linux Libertine O}
\usepackage[Latin,Arabic]{ucharclasses}
\setTransitionsForLatin{\begingroup\englishfont}{\endgroup}
\renewenvironment{english}
{\begin{LTR}\begin{otherlanguage*}{english}}
{\end{otherlanguage*}\end{LTR}}
\renewrobustcmd{\textenglish}[1]{\LR{\foreignlanguage{english}{#1}}}
\begin{document}
هذا نص بالعربية وفيه بعض الرموز الاخرى مثلا (-، * ، + ، )
\end{document}
Output



\foreignlanguage{english}{...},\begin{otherlanguage*}{english}...\end{otherlanguage*}is better and will automatically do what you're looking for, because\englishfontwould be selected, with the added benefit of correct hyphenation. – egreg Apr 29 '15 at 11:02