You can enquire about the different translations from the UH REDCap Support. At the moment in our REDCap we have the following translations of the interface: Finnish, Swedish and French
You can translate your form with the Multi-Language Management tool:

To add e.g. Finnish translation of the interface from the system, first click on 'Add new language':

From drop-down list, choose Finnish:

And again, click 'Add Language':

After you have added the language to your project, you still need to activate the language in your project.
Remember to Save Changes when you have made changes.
If the yellow line is visible on the left, you have unsaved changes in your translation:

In order for the language to be visible for the online survey respondents, you need to activate the language on the Survey tab:

You can translate the user interface text in the User Interface tab:

The survey questions are translated in the Forms/Surveys tab.
Choose the language you want to translate, and click 'Translate':

Write your translation on the field label line.
NB! Rich Text Editor gives you more editing options:

Also the survey settings are translated on the Forms/Surveys tab:

If you need to know what language the respondent has used, you need to have field for collecting language information in your form/instrument. Language codes are their ids (e.g. en, fi and sv) and you also need to use several action tags (@language-set, @language-current-form, @language-current-survey). One action tag is @hidden-survey, so that the field is not visible for the respondents, so that they don't have to answer this question after having selected their desired language. The variable name here is preferred_language:

In Multi-Language Management, choose the field that contains the preferred language information: preferred_language and save:

Now when you look at the data you have collected, you can see in the report (in Data Export, Reports and Stats) what language the respondent has used:

If you need separate URLs for the different language versions of the survey, you can embed the language id in the survey's URL. This way the respondents can open directly the preferred language version of the survey.
First add the action tag @language-set on the language field, same as in previous example:

Then copy survey's URL in Survey Distribution Tools and add & sign and the variable name of the field where you have language choices (e.g. preferred_language) and finally = sign after the language code (e.g. sv)
The link looks now like this: https://redcap.helsinki.fi/redcap/surveys/?s=NW8PF7ANKC8FFN78&preferred_language=sv