Clean up all string literals in code and replace them with string resources

[misaochan]

Subtask of #1092

Clean up all string literals in code and replace them with string resources, to better take advantage of TranslateWiki translations. (Fairly simple task)

[RozaliiaRusskikh]

Hi there! I'm a beginner and I'm eager to contribute to this project. Could I take this issue?

[nicolas-raoul]

@RozaliyaRusskikh Yes, it is yours!
Please let us know about your progress :-)

[RozaliiaRusskikh]

Thank you! I am starting right now and I will create PR as soon as possible :)

[misaochan]

Welcome aboard, @RozaliyaRusskikh ! Let us know if you need any help. :)

[RozaliiaRusskikh]

Hello! Unfortunately I do not have enough time to work on this issue. So the issue can be reassigned.

[PavelAplevich]

@misaochan I think, all done. You can close this issue

[misaochan]

Thanks @PavelAplevich !