That is because those messages are now translated in Hindi, where missing messages for Assamese are taken from. The fix is to translate those messages into Assamese.

Namespace localisation with Hindi fall back was added in r41133 and rr41134. You can go to http://translatewiki.net and help complete the Assamese localisation. Current stats for MediaWiki core is 35%, so loads of work to be done there.

wikichaipau wrote:

We, the editors and translators of Assamese wikipedia, were at work with Assamese localization. Unfortunately, many of us are not proficient in Hindi, though all of us are proficient to some level in English. This has caused a hindrance in our translation work. We request that the fall back be English rather than Hindi.

We look forward to your help in this.

Thanks,
Chaipau

Would Bengali (bn) be a more suitable fall back language? http://www.ethnologue.com/show_family.asp?subid=90420

wikichaipau wrote:

The fallback language should be English. The users and editors of the Assamese wikipedia are likely to know English more than either Hindi or Bengali. For a partially translated http://as.wikipedia.org, users would be able to navigate better with English as the fallback language.

Thank you,
Saurav

Set fallback to Bengali in r44764.

Closed after feedback from brion.