Is pronouncing shahada (in Arabic) part of excepting Islam, or just
believing in that is sufficient?


It is not essential to make the annoucement of the Shahadah publicly. Simply saying it by oneself will cause a person to become a Muslim.

But in order to be our brother or sister in Islam, one must pronounce
their Shahadatayn, in any language, in front of the Muslims.
If they keep it secret, than Allah knows what is in their hearts, and
He will reward based on that in the Akhirah, but they will be
considered a non-Muslim from the worldly perspective.

It is not sufficient to simply pronounce the Shahadah, but one must
also have complete conviction in their heart. Know that Islam is the truth,
and hasten to make your shahadah, and do not delay.

Allah knows best.