Hi Denis,
Not all directions are reversed automatically. Beside footnotes, you might need to do it for tables, columns, pages, headers, footers, registers (index), etc. I don't recall if sorting was done properly for Arabic (needed for registers). Some trickery may be needed to get the RTL right when numbers are involved, say, for enumerations (theorems, lemma, etc.) and TOC numbering. These are not specific to Arabic, though, and should surface for Hebrew as well.
Once in a while I run into problems with misoriented parentheses or quotation marks (specially
https://en.wiktionary.org/wiki/%C2%AB_%C2%BB). This may be due to setupdirection algorithms or a side effect of how some Context commands work. One needs to document them, so a fix is uploaded.
Depending on what you want to typeset, you may need to change the numerals (for page numbers, enumerations, figure and table numbers, etc.) as well as dates, etc. Some of these are already handled via the \mainlanguage[ar] command.
Regards,
MHB