Why is it so hard to find accurate translations for languages (Japanese in this case)

So I've been learning Japanese for very close to 2 years, and I actually visited for the 2025 New Years, but no matter what I search, there's some words that I just can't find a translation for.

For example, I heard the phrase "omotose" = "おもとせ" but no matter how I retype it, I can't find any direct translation to whatt it means. I've had the same issue with other certain phrases from anime (and other sources) , where I can spell it out using Japanese letters (Hirigana/Katakana) without any correct answers (according to subtitles). Is the only way to learn these certain terms from talking to an actual Japanese resident/Fluent in Japanese? Or is there another way? Because this isn't the first time this has happened. From what I can recall, this has been an issue at least 10+ times (regardless of it was from anime or a youtube video of interviews, etc…).

P.S. I've never searched for this question on reddit, so IDK if it's been answered before or not.
Sorry if it already has been. 😀

P.S. 2:

I actually got complimented on my Japanese from a resident (I'm assuming because I learned the alphabet first so I knew how to sound out the syllables for their language). I did also learn Russian at an early age so I can roll my r's and pronounce the harder sounding ones without any issue (unfair advantage lol).

I also noticed that the average Japanese person can understand english if you sound it out with their alphabet… For example: instead of saying train, say Trainu, or instead of hot, say Hottu (instead of あつい or 電車 / でんしゃ)).