What are some hardships a FBI linguist/Foreign language expert experiences and has to overcome?
The obvious thing here would be the language requirements. The requirement for the highest level of linguist is native-level proficiency, which can be quite challenging if your "foreign" language really is foreign for you, i.e., you learned it in school. Or perhaps you speak the language comfortably because you learned it from family, but you haven't lived in the country where your family is from. Chances are you'll understand and speak the language very well overall, but you may struggle with contemporary slang or some regional accents or dialects. So depending on where you fall on the proficiency scale, you may or may not have trouble reaching the highest pay levels and/or full-time employment with the FBI. Written tests often contain a lot of idiom to see how well you grasp native conversation in both languages. In spoken tests, you may be asked complicated questions about personal opinion, culture, or current events to test your ability to provide nuanced answers.
Live interpreting is also part of the job in some cases. I have done this (not for the FBI), and I can tell you that it's absolutely exhausting. Interpreting requires you to be able to listen, think, and speak very carefully all at the same time, often for long periods of time, sometimes for very important people. As someone with ADHD, this isn't a job I could ever do for long periods of time, and I really hated it. My brain just doesn't work that way. But I had a friend who often took interpreting jobs at conferences where she worked 10-hour days, several days in a row. She was also exhausted afterwards and found it stressful, but she didn't struggle the same way to focus and had a much easier time with it overall.
Written translation for me was always easier. I worked as a written Russian-English translator for a long time and didn't really have any trouble with it. I also find that I work pretty quickly at written translation, usually around 1.5-2 pages per hour; but some people (still considered good translators) can take all day to translate 5-7 pages. It's worth timing yourself at translation to get a sense of where you're at. Practicing with news articles on various subjects is a good place to start.
I personally find translation from an audio recording to be a lot more challenging. The quality of the recording can make a huge difference for me, but honestly that's also true for me in English, so maybe this is just a problem I have. It's worth testing yourself on all of these things, though, to get a sense of your abilities (and whether the process frustrates you and keeps you from enjoying the job).
Circling back to the pay aspect: The federal hiring process is slow. Expect the FBI to take a long time to process your application, do background checks, administer language testing, get your security clearance, etc. So from a financial standpoint, don't bank on things happening overnight. If you're working part-time as a contract linguist, you'll probably find that you still need to pick up a side gig. Freelance translating gigs are out there, but in general I wouldn't expect the pay to be as high as the FBI's $30-40/hr range. It will depend on what your language is and what jobs you're able to land. And of course if you go the freelance translating route, don't take a job unless you're absolutely certain you can get payment in full. Unfortunately I've known too many translators who made the mistake of sending finished work to someone in a foreign country who then disappeared and couldn't be tracked down.