Sentences in newly-added languages are not properly indexed

[jiru]

To Reproduce
Steps to reproduce the behavior:

1. Add a new language to Tatoeba using the procedure described in the wiki. The new language has to have a list of sentences given as parameter to add_new_language(). In addition, sentences in that list have to have translations in some other language.
2. Try to search for sentences in the new language: it works
3. Try to search for sentences having translations in the new language: no results returned

Expected behavior
Searching for sentences having translations in the newly-added language should work right away.

Additional notes
Only sentences added prior to the addition of the language on Tatoeba are affected. Sentences added after are not affected.
Some of the broken reindexation data may fortunately get repaired at some point if some action is performed on one of the directly or indirectly linked sentences. Actions such as adding/removing audio, changing tags, ownership, unapproved status, changing language etc. will trigger a reindexation of neighbor sentences (translations and translations of translations).

Workaround
A full reindexation of Tatoeba solves the problem for existing sentences, but any sentence of languages added later on will have the same problem.

Screenshots
https://tatoeba.org/en/sentences_lists/show/171793/und/und as of writing

https://tatoeba.org/en/sentences/search?from=eng&query=B%C3%A9nin&to=guw as of writing

https://tatoeba.org/en/sentences/search?from=eng&query=B%C3%A9nin&to= as of writing