Some languages, such as Serbian, Macedonian, Lithuanian, Latvian, were removed in late 2017. This was because over time those translations ended up being unmaintained and incomplete, in addition to the fact that there were very few sales in those regions.
Latvian language is present in the last free version (5.5.5) that is still available for download
here. If you have a newer version of NetWorx, you may need to
uninstall it first before downgrading to version 5.5.5.
However, using a newer version is a much more beneficial option, as later versions have bug fixes, optimisations and are better suited to the latest OS. There may be a bit of a learning curve if you switch to English (or any other language that is present and you speak at least basics of), but most of NetWorx interface is quite clear, and the vast majority of users find their way around it pretty quickly.