[fix] Internal server error after changing UI language to BG

A placeholder has been translated to BG, issue was added 8 month ago, when BG
translation was added [1]

    msgid "Compute {functions} of the arguments"
    msgstr "Изчислете {функции} на аргументите"

The incorrect translation has been corrected here in the message files and on
weblate.

[1] https://weblate.bubu1.eu/translate/searxng/searxng/bg/?&offset=49#history
Closes: https://github.com/searxng/searxng/issues/1692


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>