[Resolved] Need help translating CPT slug without conflicts (WPML + custom code)

Background of the issue:
My default language is DE, but everything in my theme is essentially set up in EN. I don’t want to rely on WPML String Translation just for the slug, as it feels inconsistent (and would force me to store related strings in DE when the base theme is in EN). Instead, I’d like a cleaner approach, while still having:
• English: example.com/en/products/
• German: example.com/produkte/

Symptoms:
• I’m using WPML but want to avoid installing String Translation just for slug translations.
• I have a code snippet using the register_post_type_args filter to change the CPT slug based on the current language. The URL rewrites correctly (showing /en/products/ or /produkte/), but sometimes I get 404 errors or see that WPML detects multiple languages in one request (log says “Current language: de” then “Current language: en”).
• I want the CPT content to remain translatable (so each post can have an EN and a DE version), but the actual slug rewriting should stay in my own code rather than relying on WPML’s slug translation.

Questions:
Current Approach
1. Filter approach

add_filter('register_post_type_args', function ($args, $post_type) {
if ($post_type === 'product') {
$current_language = apply_filters('wpml_current_language', null);

if ($current_language === 'de') {
$args['rewrite'] = [
'slug' => 'produkte',
'with_front' => false,
];
$args['has_archive'] = 'produkte';
} elseif ($current_language === 'en') {
$args['rewrite'] = [
'slug' => 'products',
'with_front' => false,
];
$args['has_archive'] = 'products';
}
}
return $args;
}, 99, 2);

• CPT is set to “translatable” in WPML, but I’m not using String Translation for slugs.
• I flush permalinks whenever I change anything.
• The slug does update, but I occasionally get 404 errors or the wrong slug in the wrong language.

2. WPML settings
• I have “Language in directories” activated.
• The posts themselves translate fine; only the CPT slug is problematic.

Question for WPML Support
• How can I reliably translate the CPT slug without String Translation while keeping the CPT posts fully translatable in WPML?
• Why might WPML or WordPress see the language as both “de” and “en” in the same request, potentially causing a 404?
• Is there a recommended approach or best practice for using a custom code filter to handle CPT slugs in a WPML setup?

Thank you in advance for any advice or guidance on how to properly integrate custom slug rewrites alongside WPML’s translation flow!