"Ad received this month" is not translated

[btlechowski]

Follow up to #12719

Steps to Reproduce

1. Run Brave in German(DE) locale
2. Enable rewards
3. Open brave://rewards

Actual result:

Ad received this month is not translated

Expected result:

Ad received this month is translated

Reproduces how often:

Easily reproduced

Desktop Brave version:

Brave	1.17.75 Chromium: 87.0.4280.88 (Official Build) (64-bit)
Revision	89e2380a3e36c3464b5dd1302349b1382549290d-refs/branch-heads/4280@{#1761}
OS	Ubuntu 18.04 LTS

Brave	1.20.5 Chromium: 87.0.4280.67 (Official Build) nightly (64-bit)
Revision	0e5d92df40086cf0050c00f87b11da1b14580930-refs/branch-heads/4280@{#1441}
OS	Windows 7 Service Pack 1 (Build 7601.24544)

cc @rebron @mkarolin @brave/legacy_qa

[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

---

Verification PASSED on macOS 11.1 x64 using the following build:

Brave | 1.19.85 Chromium: 88.0.4324.89 (Official Build) (x86_64)
--- | ---
Revision | 4534be84786f5269fb52e9bf82643af61e2fedaf-refs/branch-heads/4324@{#1721}
OS | macOS Version 11.1 (Build 20C69)

---

Verification passed on

Brave | 1.19.85 Chromium: 88.0.4324.89 (Offizieller Build) (64-Bit)
-- | --
Überarbeitung | 4534be84786f5269fb52e9bf82643af61e2fedaf-refs/branch-heads/4324@{#1721}
Betriebssystem | Windows 10 OS Version 2004 (Build 19041.746)

• Verified the STR from the description
  

---

Verification PASSED on Samsung S10+ running Android 10 using the following build:

1.19.86 Chromium: 88.0.4324.96

---

Verification PASSED on Samsung Galaxy Tablet A running Android 10 using the following build:

1.19.86 Chromium: 88.0.4324.96