Error: Could not load language

Hello!
When I update order status for an order, I get this error:

Notice: Error: Could not load language slovenian! in /home/musictr/public_html/vqmod/vqcache/vq2-system_library_language.php on line 39

And the email message comes in english not in my default language (slovenian). I am using Opencart Version 1.5.5.1. What could be wrong? Please help!

You should use one of the fine Translation Tools. linked below, to find and fix seemengly
still MISSING Words and Phrases in your slovenian Language Translation in an easy way.
Good Luck
Ernie
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Magic Translations - OpenCart Language Translator Tool free, OC v.1.5.x:
https://www.opencart.com/index.php?rout ... n_id=24320
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Database Language Editor PRO free, OC v.1.5.x:
https://www.opencart.com/index.php?rout ... on_id=9226
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Translate Mate free, OC v. 2.0 - 2.2.x:
https://www.opencart.com/index.php?rout ... n_id=23098
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I tried that OpenCart Language Translator Tool, but it does not solve my problem. The language is already correctly translated and that error still remains. Any other solution?

The problem is still not solved, please help!