I am using the function readWorksheet from the package XLConnect to import Excel sheets in R. These sheets contain special characters (e.g., ø, õ, ú) which R does not handle very well. As far as I know, there is no "encoding" argument for the function readWorksheet as there is for the read.csv one.
Here is what I am doing so far:
data <- readWorksheet(loadWorkbook("data.xlsx"), sheet = 5)
This is UTF-8 letters table http://www.utf8-chartable.de/
I use package xlsx for excel files:
read.xlsx(file = ".xlsx", sheetName = "Arkusz1", encoding = "UTF-8", stringsAsFactors = F)
This is in polish language, but print and read.xlsx reads all letters like "ś", "ć" etc.
 "Niewłaściwa kwalifikacja memoriałowa przychodu"  "Niewłaściwe ceny transferowe"  "niewłaściwe zarządzanie relacjami z kontrahentami"
finally if you can't read xlsx, just save your excel as .csv and read csv with encoding