TYPO3 Core

`LanguageMenuProcessor` is a dataProcessor. Simply add a custom
DataProcessor as a followup manipulating the result array to your
needs.

10 = TYPO3\CMS\Frontend\DataProcessing\LanguageMenuProcessor
10 {
  as = languagenavigation
}
20 = VENDOR\Extension\DataProcessing\LanguageMenuInvalidItemProcessor
20 {
  source = languagenavigation
  as = languagenavigation
}

or simply using the item information to skip rendering a not linked item
in the template

<f:if condition="{item.available}">
  <f:then><a href="{item.link}" hreflang="{item.hreflang}" title="{item.navigationTitle}"><span>{item.navigationTitle}</span></a></f:then>
  <f:else><f:else>
</f:if>

at least as a solution until something wents into the core ;)