There's no way to opt out of rendering a component entirely.

@11ty/eleventy: 2.0.0
@11ty/eleventy-plugin-webc: 0.9.0

<!-- index.webc -->
<is-lang :lang="meta.locale.lang" is="es">
	<h1>Hola!</h1>
</is-lang>

<is-lang :lang="meta.locale.lang" is="en">
	<h1>Hello!</h1>
</is-lang>

<is-lang :lang="meta.locale.lang" is="fr">
	<h1>Bonjour!</h1>
</is-lang>

<!-- components/is-lang.webc -->
<script webc:setup>
	function isLang(inputLang, targetLang) {
		inputLang = String(inputLang).toLowerCase();
		targetLang = String(targetLang).toLowerCase();
		return inputLang && targetLang && (inputLang === targetLang);
	}
</script>

<slot webc:if="isLang(lang,is)"></slot>

Output (e.g. meta.locale.lang === 'fr'):

<h1>Hola!</h1>
<h1>Hello!</h1>
<h1>Bonjour!</h1>

Instead of the expected output:

<h1>Bonjour!</h1>

If I add the following line to the end of is-lang.webc, it looks visually correct, but of course, outputs unnecessary markup.

<span webc:if="!isLang(lang,is)"></span>

Outputs:

<span></span>
<span></span>
<h1>Bonjour!</h1>

I believe webc:if should be respected on slots so the redundant inverted "if" is not needed.

Start with the behavior shown in components/is-lang.webc and reproduce the example using the three language variants. Trace how webc:if on the slot is rendered; done means nonmatching components produce no output or placeholder markup, leaving only the matching h1.