Update providers.xml according to https://autoconfig.thunderbird.net/v1.1/gmx.de
Fix & Update French Translations

* Lire => Lu
* New translations
SK translation
Update strings.xml

Updates and improvements (RU)
Merge pull request #370

Update provider settings for GMX
Merge pull request #371

Update French translation
Merge pull request #372

Update Slovak translation
Merge pull request #373

Update Russian translation