Changes
test if {{{1}}} exists first, return empty string if template doesn't exist, per Template talk:ISO 639 name#Return empty string for codes not on the list
{{#if:{{{1|}}} | {{#ifexist:Template:ISO 639 name {{{1}}}|{{ISO 639 name {{{1}}}}}| }} | {{ISO 639 name undefined}} }}<noinclude>
{{Documentation}}
</noinclude>
{{Documentation}}
</noinclude>