Add translation support by adding po files

[Larnicone]

If you plan to make the po files available, I could contribute to the French translation for a next release. :)

[cryptowyrm]

That would be cool, thanks!

I'm currently working on a detail view for channels and playlists so that you can download either all videos or pick and choose which videos to download, and also an automatic download of newer youtube-dl versions because the version that comes with elementary / Ubuntu 18.04 is many months old, so some video websites that should be supported might not work.

Once I'm done with that I'll start making the app translatable, so it shouldn't take too long. Then there's also a bit more text in the app that needs translation :)

[NathanBnm]

I can contribute to French too ! Don't forget to make appdata.xml available for translation