I’m not sure what is going on. When I tested it, it works:

iex> "árboles más grandes" |> String.normalize(:nfd) |> String.replace(~r/[^A-z\s]/u, "") |> String.replace(~r/\s/, "-")
"arboles-mas-grandes"
iex> "los árboles más grandes" |> String.normalize(:nfd) |> String.replace(~r/[^A-z\s]/u, "") |> String.replace(~r/\s/, "-")
"los-arboles-mas-grandes"

I got the test string by copying the string from your posting, so I don’t know if that changed the encoding.

@KronicDeth this does not seem to work on all systems, at least not on mine:

"los árboles más grandes" |> String.normalize(:nfd) |> String.replace(~r/[^A-z\s]/u, "") |> String.replace(~r/\s/, "-")
"los-rboles-ms-grandes" 

There is unix library libiconv that does that. Erlang has a few wrappers, one can be installed from hex and is called iconv.

iex(1) > :application.start(:iconv)
:ok
iex(2) > :iconv.convert "utf-8", "ascii//translit", "Hubert Łępicki"
"Hubert Lepicki"
iex(3) > :iconv.convert "utf-8", "ascii//translit", "árboles más grandes"
"arboles mas grandes"

I think using iconv transliteration also replaces some of the national characters to recognized ascii replacements, I think in German ß is replaced with “ss” etc.

After you transliterated the string to most matching ascii equivalents, you can downcase it and replace whitespace with dashes.

I’m late to this party but you might find unicode_transform useful. Its a rules-based transliterator which implements currently just a few of the many CLDR transliterations. Equally it might be both too much and too little for what you need.

The transformation rules for Latin to ASCII are quite comprehensive!

@jayjun’s excellent slugify package would be my general recommendation for slugification.

Examples

iex> Unicode.Transform.LatinAscii.transform "ü ø ß"
"u o ss"

iex> Unicode.Transform.LatinAscii.transform "árboles más grandes" 
"arboles mas grandes"

iex> Unicode.Transform.LatinAscii.transform "Übel wütet der Gürtelwürger"
"Ubel wutet der Gurtelwurger"

iex> Unicode.Transform.LatinAscii.transform "Ł"                  
"L"