Ralf's German dictionary uses the normal [x] as sign for the Ach-Laut. According to the Wiktionary, the Ach-Laut is represented by [χ] (U+03C7):
lachen /[ˈlaχn̩]/, Dach /[daχ]/, Bucht /[bʊχt]/, doch /[dɔχ]/, auch /[aʊ̯χ]/
The current development version of simon doesn’t accept the [χ]. I would like to transform the [x] into [χ] in a future version of Ralf's German dictionary.