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Your LimeSurvey version: LimeSurvey Cloud Version 6.5.4  
Own server or LimeSurvey hosting: LimeSurvey hosting
Survey theme/template:  Bootswatch

==================Hi, I need to design a survey in a language currently not included to a list of supported languages (Uzbek) - please advise if this looks feasible.  I was considering the following approach:-          Use supported language (English) as base language, add some other language (e.g. Kazakh) as additional language, add Uzbek questionnaire texts under this Kazakh label, hide language switch button – so that users see the Uzbek texts and are not misguided by wrong language name  -          Don’t use built-in tips, instead provide all instructions to the question in ‘help’ sectionHowever, I will also need to translate and replace all technical messages (e.g. warnings) to Uzbek language. Is it possible to create a theme extension where these messages can be translated and stored? What is the right way to access and edit a list of technical messages?     

Of course this is feasible. Uzbek basically already exists. In theory. But it hasn't been translated yet:
translate.limesurvey.org/projects/limesurvey-6/uz/default/

So you should be able to choose Uzbek already, but all the interface texts will be falling back to the default English, if they haven't been translated yet.

Which basically is 0% at the moment for LS6 ( translate.limesurvey.org/projects/limesurvey-6/ ).

So ideally you would support the community by translating to Uzbek. I understand that it might be a little much to translate the missing 5351 strings in a short period of time.

However, many of those strings are for the backend (Limesurvey administration) and thus probably not really relevant. You probably will only need those that show in the questionnaire and I think these are quite a bit less.

What would I do: I would set this research up in Uzbek, even if the standard strings will still be in English. All the survey text and questions you can do in Uzbek anyway. And then I would look into the standard strings that you need for your survey and translate only those. You can translate them for your local installation only (if you are self hosting), which can you can implement quicker and do not have to wait for the next official update to include them. However, it would be great to give back to the community by including these strings also into the offical translation files.

Here more information on how to translate Limesurvey:
manual.limesurvey.org/Translating_LimeSurvey

Else : you can create/replace the language string via the DB (need direct access)
bugs.limesurvey.org/view.php?id=19283#c78890