Test if a translation is 'currently' available and/or a method to test for blankness

[josephwright]

The current \IfTranslation requires we know the current language. \GetTranslation does not work cleanly inside an f-type expansion and I don't think uses \unexpanded around the 'payload'. Thus there is no obvious way to do something like 'check if translating this string will give some tokens of output', as we can't do

\IfTranslationAlt{<string>}{\GetTranslation{<string>}}

or

\expanded{\noexpand\ifblank{\GetTranslation{<string>}}

I need this for addressing josephwright/siunitx#650: in Italian, I need a translation to show up that will be absent in English, so I need to know if there is a translation to handle spacing.

I guess the easiest for me would be '\IfTranslation*, which would not need the language string but would use the current language.

[josephwright]

Something as simple as

\newcommand*\IfTranslationAvailable[1]{%
  \expanded{%
    \noexpand\@trnslt@if@translation
      {\unexpanded{#1}}
      {\@trnslt@current@language}%
  }%
}

would cover my use case if it is acceptable.