I found it hard to find descriptions of how to use the international keyboards available under Linux, so spent some time trying to figure out the key combinations to produce various diacriticals and international characters. On my mx linux box, which is set to US English with the Euro key, (though the standard way of getting the Euro doesn't seem to work). These are some combinations that do work, with my compose key set to right-Alt (Alt-Gr)
á Alt+' then a
æ Alt +a then e
ç Alt+, then c
é Alt+ ‘ then e be quick with the Alt-'
€ Alt+e then =
ï Alt+i then shift + " quickly on the second set
£ Alt+Shift+L then =
ñ Alt+- then n
ö Alt+o then shift+" quickly on the second set
ō Alt+- then o
ø Alt+/ then o
œ Alt + o then e
ú Alt+' then u
ü Alt+u then shift+" quickly on the second set.