Plugin Directory: Plugin title includes a translation of another string

[SergeyBiryukov]

Background: #2919

On ​https://ru.wordpress.org/plugins/luckywp-table-of-contents/, the plugin title is translated as "LuckyWP Содержание".

However, in the translation project the string is left in English:

• ​LuckyWP Table of Contents is translated as LuckyWP Table of Contents.
• ​Table of Contents is translated as Содержание.

Random string parts shouldn't be translated like that, only the whole strings should be.

[dd32]

The main problem occurring here is that we're applying translations to translations, but that we're also running into the fact that we're translating text which isn't defined succinctly.

3929.diff​ is a rough tested variant of the current translation functions which does it slightly different

• For each translation, ordered by longest first
  • Replace the string with a placeholder (Hopefully the placeholder value never exists within an original)
• For each placeholder marker
  • Replace the placeholder with either the translation (if it exists) or otherwise, with the original (which better still exist..)

It seems like it works without problem, but it needs more testing than I have time for right now.

[dd32]

In 7899:

Plugin Directory: i18n: When translating the content of a plugins readme, don't apply the translations to other translations.

Fixes #3929.