Here is the full list of all 13 Nihongo Quest N5 Playtest achievements.
Battle 100 characters.
Battle 1000 characters
Battle 5000 characters
Battle 10,000 characters
Get a 7-day streak in Review Tower.
Get a 15 day streak in Review Tower.
Get a 30 day streak in Review Tower
Get a 50 day streak in Review Tower.
Finish learning Hiragana.
Finish learning Katakana
Finish learning Radicals
Finish learning N5
Have over 100 melon sodas. Must be claimed at shop.