I guess both the surveys are same, difference is just the langauges. Right?
One easiest way to do this is copy both csv in one folder. Then follow below steps:
1. Check in both the csv files that they do not have any duplicate sys_RespNum, if there is any respondent with same sys_RespNum, change in one of csv file with some other respondent number
2. Then open one SSI (English preferred so that you have labels in English when you do analysis, no probelm if there is no English SSI - use any one)
3. Then click on field, then data management
4. Under Get Data, click on second radio button (Data collected with a Lighthouse Studio survey) and click on three dots (...)
5. Go to the location where you have kept both the csv files and select both the files
6. Lastly click on "Get Data", then click Close
You now have all the Ids (of both the surveys) in same Lighthouse and you can do proceed with your steps to do analysis.
If there is some other difference apart from Language, you can create a csv file with sys_RespNum, Max-diff variables, design Id and version and you can add other filter variables and then do analysis.
I hope this helps.