Since you're telling the story in first person, you can say outright that the conversation is not in the language of the narration. Something like:
I understood StrangeLandian, but I spoke it badly. I should have learnt it better before I decided to travel.
To make it not boring, blend the mention of the fact the MC doesn't speak the language well with introduction of other story elements - why she's there, etc.
As far as reminders go, make them natural. The MC might stumble upon her words, her internal monologue can be something like
How do you say 'strawberries' in StrangeLandian?
She might even make mistakes:
I knew by their response that I haven't said what I intended to say, but I hoped I haven't said anything too bad.
Difference in fluency is definitely one way to remind the reader that the character is speaking and thinking in different languages.
Other tools can be more subtle - your character might compare something to the way things are at home, something might appear strange to her, she might not understand a situation, or she might be homesick.
Be aware that stilted dialogue will grow tiresome after a while. Your character might start out not speaking the language well, but she should become more proficient as the plot advances. That's not unrealistic - if you immerse yourself in an environment, you learn the language pretty quickly, even if you retain an accent. When that happens, it doesn't matter so much to the reader that the narration isn't in the same language as the dialogue, since there's less of a barrier between the character's thought and speech.
In The Left Hand of Darkness, Ursula Le Guin uses none of the tools I've just spoken of. Her MC is sufficiently fluent in the languages spoken on Winter. Occasionally there's mention of other characters speaking slower for the MC's sake, or of him struggling to understand when a lot of people are talking at once. But one tool that is used more than once, and serves best to remind that a different language is spoken is the mention and continued use of a few terms for which there is no parallel, no translation into English. There is no word for kemmer in the English language, because the biological function does not exist on earth, so the Gethen word is used. Other times the MC explains how there's no word for an Earth term, and what words he uses instead to explain it.
So, for your story, consider what words, what concepts would be missing in each language. Each time those come into play, it serves as a reminder that a different language is being spoken.