That bug probably did not affect 2.7x, only 2.8 with COW copying IDs in
threads... But root of the issue is that underlying boost i18n lib does
not support well multi-threaded access. So simply forbid any translation
from non-main thread. This *may* be an annoying limit at some point, but
doubt it will be any issue currently.
- Add blentranslation `BLT_*` module.
- moved & split `BLF_translation.h` into (`BLT_translation.h`, `BLT_lang.h`).
- moved `BLF_*_unifont` functions from `blf_translation.c` to new source file `blf_font_i18n.c`.