We depend on translators for this, these changes are therefore usually posted in our private Translators forum. If you'd like to help out with translating languages you know but aren't actively maintained anymore, you can get in touch with Crizzo, our Translations Manager. Any efforts to contribute to whatever area in phpBB would be greatly appreciated.
Download en language packs for both versions and compare them with a comparison tool like winmerge. This can give you a overview which language variables have been changed (added, deleted or modified).
For changes within a phpBB main Version (eg. 3.2) you can use the area 51 tools: Code Changes (at the bottom right)