General strength:
* Siła (pronounced SEE-wa) - This is the most common and general translation for "strength."
Physical strength:
* Siła fizyczna (pronounced SEE-wa fee-ZICH-na)
* Mocy (pronounced MO-tsi) - This is more informal and often used in the context of athletic ability.
Mental strength:
* Siła psychiczna (pronounced SEE-wa psee-KHICH-na)
* Odporność (pronounced od-POR-nossť) - This translates to "resilience" and can also be used for mental strength.
Strength of character:
* Mocny charakter (pronounced MO-tsni kha-RAK-ter) - This literally translates to "strong character."
* Odwaga (pronounced od-VA-ga) - This translates to "courage," but can also be used to describe strength of character.
The best translation for you will depend on the specific context of your sentence.