Spanish translation lacking proper Plural-Forms header

The problem lies on exporting a custom translation of Drupal 6.2 interface, in this case Spanish, built on the existing Spanish translation from Drupal.org for that language. I say custom because I have added translations for some missing items. I ceased translation to test an export/import to see if my added translations would be imported successfully without issues on a new install of Drupal 6.2. The issue seems to be with the plural versions of some strings that share the same ID. If I try to import the newly created .po, I get the following error:

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The translation file es.po contains an error: "msgstr" was expected but not found on line 5135.
The translation import of es.po failed.
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Inspection of the lines referenced here shows the following code:

These are lines 5131 through 5142. Line 5135 is the blank line after the line [ msgid_plural "The menu name can't be longer than @count characters." ]:

#:  modules/menu/menu.admin.inc:505
msgid "The menu name can't be longer than 1 character."
msgid_plural "The menu name can't be longer than @count characters."

#:  modules/menu/menu.module:23
msgid ""
"For more information, see the online handbook entry for <a "
"href=\"@menu\">Menu module</a>."
msgstr ""
"Para más información, consulte la referencia en el manual para el <a "
"href=\"@menu\">módulo menú</a>."

Further inspection inside the install of Drupal 6.2 that generated that .po export shows that, in fact, a translation had been provided for that line, but the export function failed to include it. I achieved this by searching for the string in the Translate interface section of the Administration area and noticed a translation had been provided with the downloaded Spanish translation from Drupal.org.

What are the steps required to reproduce the bug?
Add some missing translations to the existing Spanish translation download from Drupal.org and export it to a .po file. Try importing the exported .po either inside a new Drupal install or the same one that generated the .po file.

What behavior were you expecting?
Expected the import to be successful based on the options selected, either "Strings in the uploaded file replace existing ones, new ones are added" or "Existing strings are kept, only new strings are added".

What happened instead?
I got this error:

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The translation file es.po contains an error: "msgstr" was expected but not found on line 5135.
The translation import of es.po failed.
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If I manually add a translation string on the reported line to the .po file, i get the following error:

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The translation file es.po contains an error: "msgstr" is unexpected on line 5134.
The translation import of es.po failed.
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