How to apply filters to custom string?

How to apply filters to custom string?

- Yuriy Petrovskiy の投稿
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I am currently developing grading plug-in for moodle.

Some of my courses are using multilingual content filter and have strings like 

<span lang="en" class="multilang">Module 1</span><span lang="ru" class="multilang">Модуль 1</span><span lang="ua" class="multilang">Модуль 1</span>

I need to output only one of variants. Filters work for content of moodle genuine plug-ins, but I cannot find how to use filters "manually".

I had searched forum and google, but found nothing about the subject.

Could someone help me with this matter?

Thank you in advance.