TypeScript: Tiny super basic I18n

I18n.ts

class I18n {
  public translations: TranslationsInterface;

  public locale: string;

  constructor(locale: string = '', translations: TranslationsInterface = {}) {
    this.translations = translations;
    this.locale = locale;
  }

  public t(key: string, params?: { [key: string]: string }) {
    const translation = key.split('.').reduce((tmpMessages: any, part: string) => {
      if (!tmpMessages || !tmpMessages[part]) return null;

      return tmpMessages[part];
    }, (this.translations[this.locale] || {}) as LanguageTranslationsInterface);

    if (!translation) {
      return 'ERROR: No translation for: ' + key;
    }

    if (!params) {
      return translation;
    }

    return Object.keys(params).reduce((msg: string, paramKey: string) => {
      return msg.replace(new RegExp(`%{${paramKey}}`, 'g'), params[paramKey]);
    }, translation);
  }

  public toCurrency(cents: number) {
    const {
      decimals = 2,
      unit = '$ ',
      separator = '.',
      delimiter = ','
    } = (this.translations[this.locale]?.number as NumberTranslationsInterface | null)?.currency?.format ?? {};

    return unit + (Math.round(cents) / 100)
      .toFixed(decimals)
      .replace('.', separator as string)
      .replace(new RegExp(`\\d(?=(\\d{3})+\\${separator})`, 'g'), `$&${delimiter}`);
  }
}

export interface TranslationsInterface {
  [language: string]: LanguageTranslationsInterface | { number?: NumberTranslationsInterface };
}

export interface NumberTranslationsInterface {
  currency: {
    format: {
      unit?: string,
      separator?: string;
      delimiter?: string;
      decimals?: number;
    };
  };
}

export interface LanguageTranslationsInterface {
  [keyOrNamespace: string]: string | LanguageTranslationsInterface;
}

const i18n = new I18n('nl', {
  nl: {
    number: {
      currency: {
        format: {
          unit: '€ ',
          separator: ',',
          delimiter: '.',
        }
      }
    },
    hello: 'world',
    nested: {
      is: {
        bitchin: 'word',
      }
    }
  }
});

i18n.toCurrency(1200050);