Answer
This means the specific spellout rules for converting amounts into words for that particular currency-language combination simply aren't available — a third-party tool is used to perform this conversion, and not every currency-language combination is covered by it, which is why the output falls back to English for unsupported combinations rather than erroring outright. There's no configuration workaround for this; getting a specific missing combination added as a standard fix requires contacting IFS support with a case, and you would also need to supply the correct spellout rules for that specific language, since IFS itself doesn't maintain the full set of spellout rules for every possible language.