Do not create ZIM with eng,eng language metadata

[benoit74]

See https://library.kiwix.org/raw/english.stackexchange.com_mul_all_2024-05/meta/Language

Analysis: #259 requested to enhance the metadata. Implementation done in 2671642 is however a bit too naive, it create silly eng,eng metadata when we are dealing with this english stackexchange.

We must probably:

• not create eng,eng language metadata, only eng is sufficient
• not create the name / filename as mul but eng

Nota: beware that second point means a renaming of a book, which has to be handled with the book renaming procedure to avoid issues in the library and imager service (double books)