The interface message as provided above is search-result-size. The english version, provided by dev, is as follows:

$1 ({{PLURAL:$2|1 word|$2 words}})

This renders as:

1 KB (187 words)

The french translation, provided by translatewiki:

$1 ($2⎵mot{{PLURAL:$2||s}})

And renders as

207 octet (23 mots)

In both cases the initial bit is not provided directly by the translation message, instead we are passing a sizeParameter in $1 and the language handling is formatting the size as appropriate for the language. This formatter receives a number of bytes and rounds to an appropriate display magnitude. Example usage:

$ mwscript shell.php --wiki=testwiki
Psy Shell v0.12.0 (PHP 7.4.33 — cli) by Justin Hileman                                     
> $langFactory = MediaWiki\MediaWikiServices::getInstance()->getLanguageFactory();
= MediaWiki\Languages\LanguageFactory {#775}

> $msg = new MediaWiki\Message\Message( 'example', [], $langFactory->getLanguage( 'fr' ) );
= MediaWiki\Message\Message {#8774}

> sudo $msg->message = '$1'
= "$1"

> $msg->sizeParams( [ 200 ] )->text();
= "200 octet"

So this is a general problem with how byte sizes are formatted in French. English in comparison pluralizes bytes:

> $msg = new MediaWiki\Message\Message( 'example', [], $langFactory->getLanguage( 'en' ) );
= MediaWiki\Message\Message {#6696}

> sudo $msg->message = '$1'
= "$1"

> $msg->sizeParams( [ 200 ] )->text();
= "200 bytes"

These look to resolve into the size-bytes message. Curiously those do format as expected:

> (new MediaWiki\Message\Message( 'size-bytes', [], $langFactory->getLanguage( 'en' ) ))->params( [ 200 ] )->text()
= "200 bytes"

> (new MediaWiki\Message\Message( 'size-bytes', [], $langFactory->getLanguage( 'fr' ) ))->params( [ 200 ] )->text()
= "200 octets"

This simplifies down into Language::formatSize. Querying for single and plural exposes what is happening here. English is always plural, french is never plural.

> $langFactory->getLanguage( 'fr' )->formatSize( 200 );
= "200 octet"

> $langFactory->getLanguage( 'fr' )->formatSize( 1 );
= "1 octet"

> $langFactory->getLanguage( 'en' )->formatSize( 200 );                                                                                                                                                                                        
= "200 bytes"

> $langFactory->getLanguage( 'en' )->formatSize( 1 );
= "1 bytes"

The underlying problem seems to be that Language::formatComputingNumbers works roughly as follows. In our case $msg = 'size-bytes'. The size-bytes message includes a plural handler, but this code skips the normal handling and doesn't end up invoking the plural.

$text = $this->msg( $msg )->text();
return str_replace( '$1', $this->formatNum( $size ), $text );

I suspect this is some form of optimization that was applied? I would typically expect this code to do the following:

return $this->msg( $msg )->numParams( [ $size ] )->text();

In general, not a search specific problem. The language team will have to weigh in on their preferred fix.

Removing Search Platform as this isn't directly related to Search.

@Nemo_bis: is this something you could help with?

Someone could probably also ask Language-Team to investigate, they exist :)