Combination of language detection (with python package langdetect) and aspell for mutt. To autochoose the right dictionary when spellcheking.

detect_language.py

#!/usr/bin/python
from langdetect import detect, detect_langs
from sys import stdin
import argparse

parser = argparse.ArgumentParser(description="Detect the most likely language of a text. Returns a two letter language code")
parser.add_argument('-f', '--file', help="File to be checked, otherwise stdin is used")

# For the future if my pull-request is merged. A limitation to 2 langugages improves speed by a factor of 10.
# parser.add_argument('-l', '--lang', nargs='*', help="List of languages to be checked. If ommitted, all languages are checked.")
parser.add_argument('-v', '--verbose', action='store_true', help="Return the detection probabilities as a list")

args = parser.parse_args()
text = ''

if args.file:
    with open(args.file) as f:
        text=f.read()
else:
    text = stdin.read()

if args.verbose:
    lang = detect_langs(text)
else:
    lang = detect(text)

print(lang)

muttspell.sh

#!/bin/bash
# Detecting the language before passing to aspell
# Mutt adds the -x option to the comand set as ispell
# therefore the filename is $2

# Usage in neomutt:
# 1) make script executable
# 2) adjust path to detect_language.py
# 2) set ispell = "/path/to/script/muttspell.sh"

lang=$($HOME/bin/detect_language.py -f $2)
case $lang in
    de)
        lang='de_DE'
        ;;
    *)
        lang=$lang
        ;;
esac

aspell -l $lang -e -c $2