When using a language with a custom key including a slash the generated translate function keys won't match the i18n translation files.

According to the documentation:

From Hugo 0.31 you no longer need to use a valid language code. It can be anything.
#3564

When using a language config like this:

languages:
  fragen:
    languageName: Deutsch
    # ....
  en/questions:
    languageName: English
    # ....

The generated translate function will use the i18n translation file names as keys.
But executing the translation, the lookup key will be using the original languages key ("fragen" and "en/questions" respectively)
Since I cannot name the translation files including a slash, I tried replacing it with a colon, but had no success.

For me this patch works, but it is hardly a generic solution

--- a/langs/i18n/i18n.go
+++ b/langs/i18n/i18n.go
@@ -49,6 +49,7 @@ func NewTranslator(b *i18n.Bundle, cfg config.Provider, logger loggers.Logger) T
 // Func gets the translate func for the given language, or for the default
 // configured language if not found.
 func (t Translator) Func(lang string) translateFunc {
+       lang = strings.SplitN(lang, "/", 2)[0]
        if f, ok := t.translateFuncs[lang]; ok {
                return f
        }

What version of Hugo are you using (hugo version)?

$ hugo version
hugo v0.89.0-ADE966B8 linux/amd64 BuildDate=2021-11-02T10:00:18Z VendorInfo=gohugoio

Does this issue reproduce with the latest release?

Yes