Can I translate content?
No, this is currently not possible. But on our roadmap though.
A specific text is not found, what can be the reason?
Collecting translations might be disabled. Try enabling it and reloading the specific page.
Or, that text was “hardcoded” and not using the internal localization processor. If this is a text directly from Atlassian you may ask Atlassian support for this, if this is a text from a specific add-on, you may ask the add-ons vendor.