Presuming a Jew wants to observe the laws of their religion, are they obligated to learn and speak Hebrew? Or is it sufficient if they read a translation of the Torah into their local language?
The vast majority of mitzvoth do not require any language skills whatsoever. For example, you don't need to know any language to give charity, observe Shabbat, eat kosher, or shake a lulav.
Few of the mitzvoth involve speech or listening. These are general limited to prayers and similar rituals. Public Torah reading needs to be done in Hebrew (although, there is some opinion, I believe that allows sign language, but that's a huge separate topic.) Shema can be done in any language, I believe. I'm quite certain that the rest of the prayers can also be done in any language.
In brief, there is very little requirement to understand Hebrew to perform mitzvoth correctly and effectively.
As for understanding hwat to do, of course, it always helps to read texts such as the Torah in its original language as you will get the most accurate translation and understanding and appreciate it best. Any translation is bound to dilute some of the original meaning. However, these days there are numerous accurate translations of the Torah and other texts that you can use that numerous people use to understand things.