(In reply to comment #0)

current result: "jv-java"

Where can I see this? Testcase highly welcome.

Any wikis that have Extension:I18nTags would do, such as TWN:

http://translatewiki.net/w/i.php?title=User:Bennylin/test&oldid=5247690

I revised the request:
*"{{#languagename:jv-java}}" should show "Jawa (Javanese script)", while
*"{{#language:jv-java}}" should show "ꦧꦱꦗꦮ(ꦲꦏ꧀ꦱꦫ)" => basa jawa (aksara), in Javanese script, ''scriptio continua'', no white space

In addition, provisionally "jv-latn" should also be declared thus:
*"{{#languagename:jv-latn}}" should show "Jawa (Latin script)", while
*"{{#language:jv-latn}}" should show "basa Jawa (Latin)"

Thanks.

And for completion sake, there's the third script, "jv-arab" ([[w:Pegon alphabet]])

• {{#languagename:jv-arab}}: "Jawa (Pegon alphabet)"
• {{#language:jv-arab}}: "باهاسا جاوة (بيكون)" => basa jawa (pegon)

https://www.mediawiki.org/wiki/Extension:I18nTags does not explain where to find or file bug reports. (Sigh.)
CC'ing @Nikerabbit as extension author.

{{#language}} is in core. Do not use {{#languagename}} anymore.