Various user text improvements

This patch has various user interface text improvements:
* 'Mode' and 'Import mode' to 'Update mode'
* Additional explanation with 'Store downloaded files'
* Altered text of Update modes
I removed/avoided the word 'import' as it may cause confusion because data transfer from a remote server to the website is not clearly an import. Instead I used 'update' or 'translation update'. Further I avoided the use of 'string' as I consider this to be a technical name for what is usually referred to as 'translation'.