I think you can use any strings you wish. I did for my custom button set. When using JOSM they simply come in as points with the label you specified so you can add the point or way you want and then tag as you wish using standard OSM tags. For example, there are places with new road construction where a single two direction way being rebuilt as dual carriageway roads. And the construction is new enough to not show in the Bing imagery. So I have a custom OMStracker button for “begin divided” and another for “end divided”. In JOSM I use the points with those labels to know where to convert the single carriageway to a dual carriageway which is a lot more than simply setting a tag or two especially when the way is part of some route relations.
With respect to where the language translations come from, I have no idea. But if you are making your own custom button set you can use your native language.
I was wondering what would happen if I upload from OSMtracker directly to OSM, without prior editing in JOSM.
We made an overland trip of a year from Europe to South Africa and intend to continue making such trips for the next few years. That means that we get lots of opportunities to collect data in areas visited by few people, but with very little possibility for data editing. Therefore I would prefer to send the data to OSM with minimal post processing.