The DejaVu fonts version 2.31-1 indicate which scripts (writing systems) they support by means of otlayout codes like otlayout:latn for Latin. However, the codes otlayout:lao and otlayout:nko should be otlayout:laoo and otlayout:nkoo as you can see here http://www.unicode.org/iso15924/ Please fix this in all DejaVu fonts for the next release.
ISO 15924 isn't the list of OpenType script tags. This is the list: https://www.microsoft.com/typography/developers/opentype/scripttags.aspx As you can see "lao" and "nko" are the correct OpenType script tags.
Thanks. FYI, I have found a more up to date list of script codes: https://www.microsoft.com/typography/otspec/scripttags.htm
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