It isn't entirely clear how strict you want the match to be: whether any of "defr", "de fr", ",," or "de,,fr" should be accepted. Let's start by joining the valid languages:

  my $re_langs = sprintf '(?:%s)', join '|', keys %validLanguages;
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Note that wrapping the alternatives in non-capturing parens allows me to treat $re_langs as if it were an atom in the examples below.

Now, if any combination of languages, whitespace and commas is ok:

  $text =~ /^(?:$re_langs|\s|,)*\z/;
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That allows the empty string and all the examples above. To fail on the empty string you can replace the '*' in the pattern (zero or more) with '+' (one or more).

To allow any combination, but require whitespace or commas separating languages (so that "defr" is not allowed) we require each language to be followed either by a separator or end of string:

  $text =~ /^(?:$re_langs(?=\s|,|\z)|\s|,)*\z/;
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That pattern can also be made simpler and faster if the language strings always start and end with a word character:

  $text =~ /^(?:$re_langs\b|\s|,)*\z/;
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If additionally the comma is optional but cannot appear multiple times, so that "de fr" is ok but "de,,fr" is not, one way would be to extend the pattern to say precisely that:

  $text =~ m{
    ^
    (?: $re_langs \b
      | \s
      | , (?! \s*, ) # comma not followed by another comma
                     # (not even with intervening whitespace)
    )*
    \z
  }x;
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However it is probably more efficient to encode the patterns that must follow each language:

  $text =~ m{
    ^
    \s* ( ,\s* )?  # allow stuff to precede first language
    (?: $re_langs
      (?: \s+
        | \s* , \s*
        | \z
      )
    )* \z
  }x;
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Finally, if each language must be followed by a comma but the final comma is optional, and all whitespace is optional:

  $text =~ /^\s*(?:$re_langs\s*(?:,\s*|\z))*\z/;
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I hope that gives you some useful options to consider, but please keep in mind that all the examples above are untested.

Hugo