Some strings in Rewards onboarding within Welcome UI are not translated

[btlechowski]

Follow up to #13119

Steps to Reproduce

1. Clean profile
2. Run Brave in Polish (PL) locale
3. On Welcome UI (brave://welcome), open Rewards onboarding

Actual result:

Some strings in Rewards onboarding are not translated
Light theme:

Dark theme:

Expected result:

All strings in Rewards onboarding are translated

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Brave	1.19.78 Chromium: 87.0.4280.101 (Official Build) dev (64-bit)
Revision	9407c80213cda69c2b7abcb4fa8e3f74488f4956-refs/branch-heads/4280@{#1807}
OS	Ubuntu 18.04 LTS

cc @mkarolin @brave/legacy_qa @rebron @zenparsing

[btlechowski]

Verification passed on

Brave	1.19.85 Chromium: 88.0.4324.89 (Official Build) (64-bit)
Revision	4534be84786f5269fb52e9bf82643af61e2fedaf-refs/branch-heads/4324@{#1721}
OS	Ubuntu 18.04 LTS

Verified test plan from the description

German

Polish

---

Verification passed on

Brave | 1.19.86 Chromium: 88.0.4324.96 (ಅಧಿಕೃತವಾಗಿ ನಿರ್ಮಿಸಿ) (64-ಬಿಟ್)
-- | --
ಪರಿಷ್ಕರಣೆ | 68dba2d8a0b149a1d3afac56fa74648032bcf46b-refs/branch-heads/4324@{#1784}
OS | Windows 10 OS Version 2004 (Build 19041.746)

Kannada

Germany

Hindi

---

Verified passed with

Brave | 1.19.86 Chromium: 88.0.4324.96 (Versão oficial) (x86_64)
-- | --
Revisão | 68dba2d8a0b149a1d3afac56fa74648032bcf46b-refs/branch-heads/4324@{#1784}
SO | macOS Versão 10.15.7 (Compilação 19H15)

Verified STR from description for a few languages not checked on other OSes.

French - Dark	French - Light

Japanese - Dark	Japanese - Light

Portuguese - Dark	Portuguese - Light