Shouldn't Accessibility Checker tag Language as "Needs manual check"?

It's checking whether or not the language has been set, but not checking whether it's the correct language.

I agree with you: as it is now, it's pretty useless and language should be manually checked.

Same with Alt Text on graphics and hyperlinks: checks whether anything is in the Alt values, not that the Alt Text is correct. You can put gibberish in the Alt Text field and ALL of the checkers will pass it.

And ditto with tags, too. It checks whether at least one tag is in the document, not whether everything is tagged or that they are the right tags. We frequently see client PDFs like these:

• Everything is tagged <P> without any  headings, lists, etc.
• Only 1 paragraph is tagged, the rest of the document is untagged (not artifacted).
• Compound tags like lists contain only <L> and not the <LI> <Lbl> <LBody> combo.

All of the checking software, not just Adobe Acrobat's, are merely automated programs that can evaluate pass/fail  yes/no  0/1  situations. That's all the programmers can program the checkers to do. There's no "I" in the "AI" they use.

So most of the checkers are fairly useless if that's the only method used to check a PDF for compliance. Same for websites where vendors claim they can check everything for compliance on a multi-million page site.

Compliance checks must also include humans who can provide "RI" -- Real Intelligence and knowledge about accessibility.