I have the same problem, and no idea why this is happening. My workaround is to simply add a language section to my pack.mcmeta, which is quite possibly a bad idea, but it works:
{
"pack": {
"pack_format": 1,
"description": "OpenComputers"
},
"language": {
"en_US": {
"name": "English",
"region": "United States"
}
}
}
Edit: oops, sorry for the semi-necro, I found the topic via google and didn't check the date.