fix: slug transliteration assuming the language is always en

[SychO9]

Changes proposed in this pull request:
Right now slugs are generated from transliterated title, but always assuming en language. So if a forum is using another language with different characters like Chinese, transliteration doesn't work.

This PR passes the forum's current locale code to the slug method of laravel, to properly transliterate into the forum's language.

Reviewers should focus on:
The problem however is if a forum uses multiple languages it won't work because laravel's slug doesn't detect language on its own, so for example on discuss if someone created a discussion with a Japanese title, the slug will not work, but I believe this is still better than not.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.