Optimize Language Mapping Fields to Generate Only for Sales Channel Languages

### User story

Currently, translated fields in Shopware generate language mapping fields for all languages, regardless of their relevance to specific sales channels. This results in redundant mappings and increased resource usage, which can affect performance and efficiency. To address this, I propose an enhancement to generate language mapping fields and indices only for languages associated with active sales channels.

Proposed Solution:

Modify the system to generate language mapping fields and index only for languages used by sales channels.

### Acceptance criteria

Ensure that only these relevant language mappings are indexed, reducing redundancy and optimizing resource usage.

The issue names no files, tests, or entry points. Start by locating the language-mapping generation and indexing logic, then trace how active sales-channel languages are determined. Done means mappings and indices are generated only for languages associated with active sales channels, with redundant mappings removed.

Written by the indexing model from the issue text.