When using the LanguageAnalyzer class for registering an Analyzer and using it's property StemExclusionList, this list isn't recognized by elastic search because there is a space in the generated Json which doesn't belong there:
[JsonProperty("stem_exclusion ")]
public IEnumerable<string> StemExclusionList { get; set; }
This is the relevant snippet from the elastic search source. You can see it on the property parameter after "stem_exclusion".
I hope this helps.