Hi!
Please follow this instruction –
1) Create a directory supsystic-membership-lang in directory of your theme.
2)Create files with required translation and add them to this folder.
3) The example of the file name – sl_Sl.po
4) Or you can place your files by this path – /wp-content/languages/plugins/. But in this case files should be name like this – sm-sl_Sl.po