set inputStringWithLigatures to "ꜲꜳÆæꜴꜵꜶꜷꜸꜹꜺꜻꜼꜽffffifflfiflŒœꝎstꜨꜩᵫꝠꝡ" & "LJLjNJNjnjDZDzdzIJij"
set aRes to removeLigaturesFromString(inputStringWithLigatures) of me
–>　　"AAaaAEaeAOaoAUauAVavAVavAYayffffifflfiflOEoeOOstTZtzueVYvyLJLjNJNjnjDZDzdzIJij"

on removeLigaturesFromString(inputStringWithLigatures)
　　set theString to current application’s NSString’s stringWithString:inputStringWithLigatures
　　
　　# Convert what may be done applying transform "ÆæffffifflfiflŒœᵫ" & "LJLjNJNjnjDZDzdzIJij"
　　set inputStringWithLigatures to (theString’s stringByApplyingTransform:"Latin-ASCII" |reverse|:false) as text
　　
　　# Treat the remaining ligatures
　　set searchStrings to {"Ꜳ", "ꜳ", "Ꜵ", "ꜵ", "Ꜷ", "ꜷ", "Ꜹ", "ꜹ", "Ꜻ", "ꜻ", "Ꜽ", "ꜽ", "Ꝏ", "Ꜩ", "ꜩ", "Ꝡ", "ꝡ"} — if you find others, add them here
　　set replaceStrings to {"AA", "aa", "AO", "ao", "AU", "au", "AV", "av", "AV", "av", "AY", "ay", "OO", "TZ", "tz", "VY", "vy"} — if you find others, add them here
　　set saveTID to AppleScript’s text item delimiters
　　
　　considering case
　　　　set i to 0
　　　　repeat with lig in searchStrings
　　　　　　set i to i + 1
　　　　　　set AppleScript’s text item delimiters to {lig}
　　　　　　set inputStringWithLigatures to text items of inputStringWithLigatures
　　　　　　set AppleScript’s text item delimiters to {item i of replaceStrings}
　　　　　　set inputStringWithLigatures to inputStringWithLigatures as text
　　　　end repeat
　　end considering
　　
　　set AppleScript’s text item delimiters to saveTID
　　
　　return inputStringWithLigatures
end removeLigaturesFromString