[5.1] Languagefilter: Fix implementation of subscriberinterface

[Hackwar]

Summary of Changes

Unfortunately I made a mistake in #42692 by not converting all events to the new SubscriberInterface, making the languagefilter plugin fail in some areas. This PR fixes that.

Testing Instructions

1. Enable the languagefilter plugin
2. Edit your profile, for example in the backend
3. Click on save.

Actual result BEFORE applying this Pull Request

Error

Expected result AFTER applying this Pull Request

No error.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[dautrich]

I have tested this item ✅ successfully on d1068f7

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[joomdonation]

I have tested this item ✅ successfully on d1068f7

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[joomdonation]

RTC

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[LadySolveig]

Thank you @Hackwar 🚀 and also to the fast and diligent testers @dautrich and @joomdonation