While all five sublevels of level 1 are grouped together in Debian Installer status pages, it is recommended to work on level 2 translations after completing sublevels 1 and 2. Sublevel 3 can then be started after completing level 2. Sublevel 4 can be started after completing Level 3 and, finally, sublevel 5 can be started after completing Level 4.
The resulting suggested work order is the following:
Level 1 sublevel 1
Level 1 sublevel 2
Level 2
Level 1 sublevel 3
Level 3
Level 1 sublevel 4
Level 1 sublevel 5