"Insufficient storage - The device does not have enough space to download…" error only when downloading a language pack in Swiftkey
Turns out this was a bug with SwiftKey and was fixed in version 6.3.6:


  Fixed: ‘Insufficient storage’ error message will not appear when downloading languages


Before that version, I did a workaround by restoring a backup of SwiftKey's data using Titanium Backup (requires root) from another device with SwiftKey and the language pack already downloaded and installed. This will however, keep the settings and logged-in user data of the source device, so you'll to log-out the existing user and edit the settings after you have restored the SwiftKey data.