Fix languages covered by M4Tv2 (#28019)
* correct language assessment + add tests
* Update src/transformers/models/seamless_m4t_v2/modeling_seamless_m4t_v2.py
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
* make style + simplify and enrich test
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Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
