Hey, Kombmail!
When it comes to singing in a different language, and even in English really, it's a good idea to sit down and deconstruct the lyrics word by word. When I sing in German, Italian, Japanese or Korean, I write my own phonetic translations down to help me achieve a more accurate pronunciation. It can be time-consuming to do but if you're noticing that it's a problem, this is a great way to do it
The English language also has a crazy amount of diphthongs which can challenge your pitch and tone. For example, the word "day" with an American accent comes out as "day-ee" and if you sing the "ee" more, it can be quite nasty if it's not in the stylistic prescriptions of the song. What you would like to work towards is pronouncing it more as "daye", rhyming with "aye", and putting that "ee" at the very end of the note so it doesn't adjust the rounder sound of the voice. Your tone will be much more consistent if you make sure you pronounce each lyric clearly and with conviction.
Alongside fixing tone, you can work on your pitch by reading sheet music for your songs or aurally working out the notes on a piano. This can help you keep in tune and know exactly what you're singing. Visualising the actual melody itself is an effective tactic to improve the movement in your voice so I would highly recommend it.
Ariana Grande is a bit of a tricky choice vocally to sing so perhaps consider either choosing a different artist or reducing her music's complexity to more manageable pitches. Rather than singing say a crazy run with six or seven fast pitches in between, aim to find a note in that chord that is more sustainable and easy to perfect first before you work towards the runs. For the HSC, also consider singing in Hindi if you're more comfortable with it. This will maximise your performance and can give you more confidence. Hope that helps!
Angelina