Fix hardcoded strings

This integrates commits proposed by @badfiles in #138 and #139 with some improvements on the original code to facilitate the work of translators.

Note: currently we use SI-compliant kilobytes to display the maximum file size (i.e. 1K = 1'000 bytes) which is somewhat unusual; I would be tempted to switch to the more standard 1024-based KB (or KiB), what do you think ?