feat: Improve required steps and mention improvements (#2108)

Add required steps to successfully remove the Language Pack after
migration.

Mention changes in the process starting with Shopware 6.7.7.0 and
Language Pack 5.37.1

Author: dgrothaus-sw

resources/references/upgrades/core/translation/language-pack-migration.md

@@ -39,7 +39,7 @@ Starting with Shopware **6.7.3.0**, translations are managed directly in Shopwar
 - Nothing changes.
 - To install additional languages, use the CLI command:
 
-```bash  
+```bash
   bin/console translation:install --locales <locale-code>
 ```
 
@@ -55,10 +55,16 @@ Example: `bin/console translation:install --locales it-IT,fr-FR` will install It
 
 2. The command uses the **same source ([translate.shopware.com][shopware-translations])** as the [Language Pack plugin][language-pack-plugin] but is
    updated more frequently. So it's essentially identical – or even more up to date!
-3. You can safely uninstall and remove the Language Pack plugin. Your **custom snippets** created in the Snippet module
-   remain intact since they are saved in the database.
-4. Make sure that all languages you need are **active** in the Administration:
-   `Settings → Languages`
+3. Make sure that all languages you need are **active** in the Administration: `Settings → Languages`
+4. Create base snippet sets for used languages
+   - If you're on Shopware **6.7.7.0** or later, this is done automatically.
+   - If you're on Shopware **6.7.6.0** or earlier, for each language in use, create a base snippet, e.g. `BASE en-US` for English (US).
+5. Change all sales channel domains to use the base snippet sets.
+   - If you're on Language Pack **5.37.1** or later, this is done automatically.
+   - If you're on Language Pack **5.37.0** or earlier, open each sales channel, scroll down to the domains and change the
+     snippet set from `LanguagePack` to `BASE`, e.g.: `LanguagePack en-US` to `BASE en-US`.
+6. It is recommended to uninstall and remove the Language Pack plugin after `translation:install` succeeded for all locales.
+   Your **custom snippets** created in the snippet module remain intact since they are saved in the database.
 
 ## New installations
 
@@ -70,6 +76,14 @@ Example: `bin/console translation:install --locales it-IT,fr-FR` will install It
 
 - Additional details about the new translation handling are available in the [integrated language handling][translation-system] guide.
 
+## Common problems and troubleshooting
+
+### Can't remove Language Pack: a foreign key constraint fails (\`shopware\`.\`sales_channel_domain\`…
+
+Starting with **Shopware 6.7.7.0** and **Language Pack 5.37.1**, the migration process was improved. If you encounter this issue,
+please update Shopware to >= 6.7.7.0, Language Pack to >= 5.37.1, remove the translation files created from running the
+command and run the command again. Or follow the updated migration guide.
+
 [translation-system]: ../../../../../concepts/translations/built-in-translation-system.md
 [language-pack-plugin]: https://store.shopware.com/en/swag338126230916f/shopware-language-pack.html
 [shopware-translations]: https://translate.shopware.com