TYPO3 Core

config.language_alt is described in TSref as follows:
If “config.language” (above) is used, this can be set to another language key which will be used for labels if a label was not found for the main language. For instance a brazil portuguese website might specify “pt” as alternative language which means the portuguese label will be shown if none was available in the main language, brazil portuguese. This feature makes sense if one language is incompletely translated and close to another language.

The following snippet should result in a dutch translation (since it's the language_alt for missing german labels) but still shows the english original:

page.config {
    language         = de
    language_alt     = nl
}

page.5 = TEXT
page.5 {
  value = This is English!
  #lang.de = Das ist Deutsch!
  lang.nl = Het is Nederlands!
}

If page.5 was a Extbase plugin using fluid translation viewhelpers this would work though.

I used 6.2.15 when testing the snippet above.

This issue seems still present on 10.3.0-dev; tested with the same snippet on comment 2.